NATO officials say they are closely watching Russia's biggest war games since 2013 with many unnerved about what they see as Moscow testing its ability to wage war against the West.

NATO believes the exercises - officially starting on Thursday in Belarus, the Baltic Sea, western Russia and the Russian exclave of Kaliningrad - are already under way. It says they are larger than Moscow has publicised, numbering some 100,000 troops, and involve firing nuclear-capable ballistic missiles.

Codenamed Zapad or "West", NATO officials say the drills will simulate a conflict with the US-led alliance intended to show Russia's ability to mass large numbers of troops at very short notice in the event of a conflict.

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Lithuania's Defence Minister Raimundas Karoblis voiced widely felt fears the drills risk triggering an accidental conflict, or could allow Moscow to leave troops in neighbouring Belarus.

"We can't be totally calm. There is a large foreign army massed next to Lithuanian territory," he told Reuters news agency.

Some Western officials - including head of the US Army in Europe, General Ben Hodges - have raised concerns that Russia might use the drills as a "Trojan horse" to make incursions into Poland and Russian-speaking regions in the Baltics.

The Kremlin firmly rejects any such plans. Russia says some 13,000 troops from Russia and Belarus will be involved in the September 14-20 drills, below an international threshold that requires large numbers of outside observers.

Moscow says it is the West that threatens stability in Eastern Europe because the US-led NATO alliance has put a 4,000-strong multinational force in the Baltics and Poland.

With Russia's seizure of Crimea in 2014 and its intervention in Syria's war in 2015, NATO is distrustful of the Kremlin's public message.

In Crimea, Moscow proved a master of "hybrid warfare", with its mix of cyberattacks, disinformation campaigns, and use of Russian and local forces without insignia.

One senior European security official said Zapad would merge manoeuvres across Russia's four western military districts in a "complex, multidimensional, aggressive, anti-NATO exercise".

"It is all smoke and mirrors," the official said, adding the Soviet-era Zapad exercises that were revived in 1999 had included simulated nuclear strikes on Europe.

NATO officials say they have been watching Russia's preparations for months, including the use of hundreds of railcars to carry tanks and other heavy equipment into Belarus.

As a precaution, the US military has moved 600 paratroopers to the Baltics during Zapad and has taken over guardianship of the airspace of Lithuania, Latvia and Estonia, which lack capable air forces and air defence systems.

NATO'S next move?

Russia's military show of force raises some uncomfortable questions for the alliance because NATO cannot yet mass large numbers of troops quickly, despite the United States' military might, NATO officials and diplomats said.

NATO, a 29-nation defence pact created in 1949 to deter the Soviet threat, has already begun its biggest modernisation since the Cold War, sending four battalions to the Baltics and Poland, setting up an agile, high-readiness spearhead force, and developing its cyberspace defences.

But NATO has deliberately taken a slow approach to its military build-up to avoid being sucked into a new arms race, even as Russia has stationed anti-aircraft and anti-ship missiles in Kaliningrad, the Black Sea and Syria.

"The last thing we want is a military escalation with Russia," said one senior NATO official involved in military planning, referring to Zapad.

In the event of any potential Russian incursion into the Baltics or Poland, NATO's new multinational forces would quickly need large reinforcements. But a 40,000-strong force agreed in 2015 is still being developed, officials say.

During Zapad, NATO is taking a low-key approach by running few exercises, including an annual sniper exercise in Lithuania. Only non-NATO member Sweden is holding a large-scale drill.

With 19,000 troops involved, Sweden will simulate an attack from the east on the Baltic island of Gotland, near the Swedish mainland.

"The security situation has taken a turn for the worse," Micael Byden, the commander of the Swedish Armed Forces, said during a presentation of the three-week-long exercise.

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Sweden, like the Baltics, Poland and much of the West, has been deeply troubled by Russia's 2014 annexation of Crimea and its support for rebels in eastern Ukraine.

"Russia is the country that affects security in Europe right now with its actions - the annexation of the Crimea and continued battles in eastern Ukraine - so it is clear that we are watching very closely what Russia is doing," Byden said.

Around 1,500 troops from the United States, France, Norway and other NATO allies are taking part in the exercise dubbed Aurora.

NATO generals say the Aurora exercise is not a response to Russian exercises that start on Thursday.

NATO Deputy Supreme Allied Commander Europe James Everard said there was no need to mirror Zapad. "It's not a competition," he said during a visit to NATO forces in Latvia.