NATO has put Moscow on notice that it will be keeping a close eye on a major military exercise with Belarus next week, in a region still on edge after Russia's annexation of Crimea in 2014.

Similar drills in the past included a simulated invasion of Poland by tens of thousands of Russian troops culminating in a nuclear strike on Warsaw, and the coming show of force, codenamed "Zapad 2017" (West 2017) has sparked months of speculation and fears along NATO's eastern flank.

Observers say that while there is little chance of Russia using the exercise as cover for an actual invasion, there are concerns about what troops and equipment it will leave behind afterwards.

Moscow has said about 12,700 Russian and Belarusian troops will take part in the exercises, to be held in Belarus and Russia's exclave of Kaliningrad from September 14 to 20, but Lithuania and Estonia have put the figure as high as 100,000.

"We are concerned about the nature and the lack of transparency of the exercise," Estonian Prime Minister Juri Ratas said Wednesday on a visit to NATO troops at the Tapa base in his country's north with alliance chief Jens Stoltenberg.

- 'Defensive' manoeuvres -

Stoltenberg himself poured doubt on Russian claims about troop numbers in July, saying that based on past experience "we have every reason to believe that it may be substantially more troops participating than the official reported numbers".

Russia says the exercises are "purely defensive", with an "anti-terrorism focus" and an "artificial" enemy, but the Belarus army chief has given a rather different scenario, in which his forces respond to an attempt at "destabilisation" by a coalition of Western countries.

To counter growing Russian assertiveness in recent years, NATO has posted around 4,000 troops in the three Baltic countries and Poland -- the biggest reinforcement in Eastern Europe since the end of the Cold War.

Western allies have been urging Moscow for months to be more open about the Zapad exercises and to allow expert observers to key parts of the event, particularly briefings on the scenario being used.

Three NATO observers have been invited to "distinguished visitors" days at the end of the exercise, but Stoltenberg warned on Wednesday that Russia was failing in its obligations under international agreements on military exercises by limiting observers' access to the drills.

But the alliance is taking steps to follow the operations, with or without Russian cooperation.

"The means will obviously be put in place by NATO to watch what is happening. That's fair enough," a diplomatic source told AFP on condition of anonymity.

- High-tech radar -

NATO and the US have powerful airborne radars deployed in drones that they can use to track the movement of tanks, trucks and other equipment on the ground from as much as 200 to 300 kilometres (120 to 180 miles) away, Brooks Tigner of IHS Jane's Defence Weekly told AFP.

This synthetic aperture radar "can switch its beam in microseconds from one target to the next and start calculating and use algorithms to determine how everything is moving... that of course will reveal to NATO what kind of exercise this is," he said.

In an anti-terrorism scenario all the troops would converge on a single small target, Tigner said, though he dismissed the idea as "absurd".

"If you're going to do a terrorist scenario, you don't need 13,000 -- that's an awful lot of soldiers to make a move against terrorists," he said.

"It's quite difficult to imagine what kind of terrorist scenario would require numbers like that."

At the Tapa base, French Colonel Olivier Wache said Zapad would not change the joint training exercises between British, French, Danish and Estonian troops that began in the spring.

"We have the means to respond if the Russians really want to cross the border, but this is not at all what we expect," Wache said.

But the diplomatic source warned of "genuine concern" within the alliance that the exercises would reinforce a more aggressive posture from Russia "or even be used to install Russian military hardware even closer to NATO countries".

Tigner agreed, saying that Moscow was well aware that an attack on a NATO member would bring a swift and serious response from the alliance, but that Zapad will give it an opportunity to show it still has considerable forces at its disposal and can keep the West on edge.

"The bigger concern is how many troops and weapons, et cetera that Russia would leave in Kaliningrad -- that makes everyone nervous, and indeed how many troops it might leave in Belarus," he said.

Source: AFP