Belarus allowed itself to be used as a launch pad for Russia’s February 24 invasion of Ukraine but has not joined the fighting directly.

Analysts say Lukashenko would have no choice but to comply if Russian President Vladimir Putin demanded he enter the war, at a moment when Moscow is reeling from a series of defeats and facing unprecedented public criticism of its generals’ failings.

But they are sceptical that Belarus’s intervention would make much of a difference. Its armed forces total just 48,000 personnel, according to the International Institute for Strategic Studies, and have not fought a war in more than 30 years of independence since the collapse of the Soviet Union.

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“It’s not exactly a combat-tested armed force,” said Samir Puri, author of Russia’s Road to War with Ukraine.

He said, however, that the risk of intervention by Belarus could force Ukraine to strengthen security in the north of the country, drawing forces away from the front lines with Russia in the south and east.

Zelensky’s call for foreign observers is a sign that Ukraine takes the risk seriously but it may not be diplomatically feasible, Puri said. It is unclear who would provide such a force, as Moscow would veto any UN role and Nato or EU observers could be drawn into clashes with Russian forces.

The Belarusian defence ministry did not immediately respond to a request for comment. On Tuesday it said the deployment with Russian troops was a defensive measure “aimed at responding adequately to actions near our borders”.

Belarus shares borders with three Nato members, a factor that may also be part of Putin’s calculations as he seeks to draw his ally into the war.

“It brings him much closer to Nato’s borders. Putin can then say: ‘I’m bringing the war to you. Do you really want it?’ What happens if a missile goes astray?” a senior European official said.

Lukashenko has not specified the size and role of the joint force he announced on Monday, though he said then that he expected the arrival of thousands of troops on Belarusian territory.

Not all the evidence suggests the Belarus military is on the verge on joining the fight.

The Belarusian Hajun project, which monitors military movements, said there is evidence of equipment moving in the other direction, including the transfer to Russia of at least two trains with 28 Belarusian tanks.

Pavel Slunkin, a former Belarusian diplomat now at the European Council on Foreign Relations, said there were no signs of Russia pouring forces into the country to reinvade Ukraine from the north, as it did at the start of the war in February.

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He said he did not believe Lukashenko was on the point of committing his forces to fight alongside Russia’s, but he may be readying himself for that eventuality.

“Maybe he hasn’t decided it yet but he understands that this could happen and in this scenario it’s better to have the army prepared.”

Slunkin said Lukashenko, who survived mass protests with Russian help in 2020 and depends on Putin both politically and economically, would be in no position to withhold military support if the Kremlin required it.

“His guarantee of keeping his power very much depends on Putin,” he said. “Lukashenko can’t survive without Russia’s support and without repression. His dependence is so deep, he has almost no space for manoeuvre.”