Alliance sources fear that Russia could launch attacks in the northernmost NATO member states involving some 600,000 troops.

Satellite images have emerged showing a build-up of Russian forces just a few kilometres from the Finnish border. There is evidence that Moscow is building troop accommodation, aircraft infrastructure and other new facilities at key military bases.

The signs that Moscow is sending weapons and troops to the area come in the wake of allegations, including by Finland's prime minister and German intelligence, that Vladimir Putin is preparing for a protracted conflict with the West.

A Finnish government report said Helsinki considered a possible attack on Finland, Norway, Sweden and the Baltic states.

“Russia is intensifying its military presence and activities in the Northwest in all operational environments as quickly as possible,” the report warns.

NATO sources said Moscow was rehearsing an attack on the bloc's eastern flank and had outlined a threat assessment for places that could be targeted.

A coordinated attack involving a number of Russian units could strike the Norwegian coast, southern Finland and the Lapland region, the Swedish island of Gotland and even enter Estonia, Latvia and Lithuania simultaneously, the sources said.

Experts suggest, however, that Putin is more likely to opt for small-scale attacks aimed at causing chaos in NATO, which will be forced to consider whether to honor its mutual defense pact or allow Russia to seize territory to avoid a full-scale European war.

In 2017, Moscow conducted seven days of military exercises codenamed Operation West, and which effectively simulated attacks on Eastern European states.

The maneuvers, which according to some Western estimates involved some 100,000 soldiers as well as thousands of armored vehicles, were aimed at taking over the fictional country of Veshnori - with a topography similar to that of the Baltic states.

"The Russians have not abandoned their plan of attack, but want to carry it out after the war in Ukraine. According to our information, the plan still exists and the Russians have not given up their desire to carry it out," a source said.

Describing in detail a possible battle plan along the lines of what Moscow has been rehearsing in exercises, the NATO sources said Russia could attack its western neighbors by land, sea and air.

This could include sending airborne forces into Finnish Lapland, where they could attempt to establish a buffer zone and seize strategic sites such as Ivalo airport.

Moscow could also launch an air attack from the Kola Peninsula and send airborne troops into the south of the country. It is said that missiles could also be aimed at Helsinki.

Russian forces could also launch an attack on the Swedish island of Gotland and try to penetrate Estonia and Latvia.

A breakthrough in Lithuania would probably aim at seizing and securing a buffer zone around the Suwałki Corridor linking Russia with the militarized exclave of Kaliningrad.

However, this threat assessment, which is seen by experts as a worst-case scenario, is based on intelligence information and where Russian army units are currently based.

Ed Arnold, senior fellow for European security at the Royal United Services Institute, said such a large-scale operation would spread the forces available to Russia too thinly.

"It would be very difficult for Russia to mount a larger-scale attack against multiple targets on NATO's northern and eastern flank. There is, however, a significant possibility of a smaller, more targeted operation organized by Russia against NATO that would aim to inflict political damage on NATO. This would put NATO in a difficult situation in terms of how to respond, whether it would meet the conditions of Article 5, etc. That is the only way they could try to attack NATO," he explained.

"The Russian scenario could be as small as going into the gap and taking over several miles of territory, widening the gap by several miles on each side. That could sow division and dissent within NATO - on the one hand, it's a military incursion into NATO territory that needs to be protected, but there will be others who will say do we want to risk a war with nuclear force over such a small piece of land," he added.

Arnold said it was “likely” that such an operation would take place on a small scale, but did not rule out the possibility of a larger-scale conflict.

He pointed out that before 2022, “many would have said a full-scale operation in Ukraine was foolish, but Putin went ahead and did it anyway.”

Last month, Finland's deputy defence chief, Lieutenant General Vesa Virtanen, warned that Russia's actions along the border suggested the Kremlin was “deliberately testing NATO's unity” to see if it would trigger Article 5, the alliance's collective defence clause.

Virtanen told the German newspaper Velt that Russia has been testing Article 5 with hybrid warfare tactics, including cyberattacks and mass cross-border migration, and is now erecting new equipment to deploy Russian troops along the border.

During the war, about 20 000 soldiers and about four on-duty brigades were deployed there, and now we see Russia building new infrastructure and, as soon as it can, more troops in that region, Virtanen warned.

Major Juha Kukola, a professor at the National Defence University of Finland and a former platoon commander in the Finnish army, pointed to an important indicator of Russian readiness in a warning late last month.

If you see them building new railway stations or repairing old ones, you'd better start paying attention, he noted.

Recent satellite images appear to show vehicle infrastructure and upgraded air bases.

In Petrozavodsk, about 100 miles from the Finnish border, recently built warehouses are visible, which experts assume are storage halls for armored vehicles.

In Kamenka, about 35 miles from the Finnish border, more than 130 military tents have reportedly been erected since February to accommodate about 2,000 troops.

Photographs from the far north show that the previously closed Severomorsk-2 air base is now operational, with renovation work apparently completed in recent years.

Several helicopters can be seen parked in bays alongside the airstrip, which is 110 miles from the Finnish border and less than 70 miles from Norway.

Meanwhile in Olenia, about 90 miles from Finland, the airbase is said to be fully active, with Kiev saying Russian bombers stationed there are carrying out raids on targets in Ukraine.

While the bulk of Russian forces are now concentrated in Ukraine, Putin is reportedly turning his attention to rebuilding forces in northwestern Russia.

The Kremlin plans to build a new army headquarters in the city of Petrozavodsk, about 100 miles from the Finnish border, which will be able to oversee tens of thousands of soldiers in the coming years.

NATO sources say Moscow could assemble an offensive force of 600,000 soldiers near the Finnish and Baltic borders, with ranks bolstered by soldiers returning from the front after the end of the war in Ukraine.

Ian Bond, deputy director of the Centre for European Reform, said that even if a full ceasefire were agreed in Ukraine, Putin's aggression was unlikely to be reduced.

"Nobody in their right mind wants to think that a European war is around the corner again. But the reality is that more and more European intelligence officials are telling us so. Whether it happens in three, five or ten years, what they are saying is that the idea that peace in Europe will last forever is now in the past," he noted.

Ruslan Pukhov, director of the Moscow-based Center for Analysis of Strategies and Technologies, warned, "When the troops come back [from Ukraine], they will look across the border to a country they consider an adversary. The logic of the last decade suggests that we are in for some kind of conflict with NATO.

In a speech before the U.S. Senate last month, General Christopher Kavoli, commander of U.S. forces in Europe, said, "The Russian military is recovering and growing at a faster rate than most analysts expected. In fact, the Russian army is larger today than it was at the beginning of the war."

Russian defense spending has risen to 6 percent of GDP this year - up from 3.5 percent at the start of the Ukraine war in 2022.

RUSI's Ed Arnold added that he is "worried about policymakers because they say Russia won't be ready for a large-scale broader conflict in Europe for several years.

“But something could happen tomorrow that could be an accident or a miscalculation that could trigger a much broader conflict,” he explained.

NATO, facing threats of funding cuts and accusations of dereliction of duty from President Trump and his officials, has vowed to “ramp up” its contribution to global security as the Russian threat looms.

In recent years, the military alliance has continued to increase its battlefield readiness with intensive military exercises on its eastern flank, including the annual DEFENDER exercises currently underway in the region.

In southern Finland, the army has reportedly practiced blowing up bridges in the event of an invasion.

In the midst of the war in Ukraine, Putin announced that his goal was to restore all of Russia's “historical territories,” and compared himself to Peter the Great, the tsar who waged war against Sweden.

Given the way Moscow has conducted its campaign in Ukraine, the Finnish government's report reportedly warns that attacks against civilians are not ruled out.

“Russia has shown in Ukraine that it is prepared to use large-scale military force against vital objects of society and civilians to achieve its political goals,” the report said.

In March, a report by Germany's Federal Intelligence Service (BND) warned that Putin was preparing for conflict with NATO.

By 2030, the BND estimated, Russia could be fully prepared for “large-scale conventional war.”

“Russia sees itself in a systemic conflict with the West and is ready to implement its imperialist goals through military force, even beyond Ukraine's borders,” the report predicted.

Finland's accession to NATO on April 4, 2023, which expanded the alliance's border with Russia by more than 800 miles, sparked fury in Moscow.

In response to Russia's neighbor joining the bloc, Putin announced the creation of a “Leningrad Military District” near the Finnish border and the deployment of additional troops to the area.

Last month, the former Russian president warned that NATO's newest members are now potential targets for nuclear retaliation if Moscow decides.

Dmitry Medvedev, who has emerged as one of Russia's most outspoken anti-Western hawks, appeared to be referring to Sweden and Finland, the last two countries to join the Western military alliance.

If it came to conflict, nuclear weapons would not be ruled out, the state news agency TASS reported.

The status of non-aligned states gives them (Finland and Sweden) certain international privileges, given their geopolitical position and many other factors, Medvedev stressed.

“And now they are part of a bloc hostile to us, which means they automatically become a target for our armed forces, including potential retaliatory strikes and even a nuclear component or preventive measures within the framework of military doctrine,” he pointed out.

Last week, Moscow also warned Britain against deploying a “coalition of the willing” in Ukraine, saying it could lead to a third global nuclear war. | BGNES