Belarus on Monday announced joint military drills with close ally Russia on its southern border with Ukraine and accused the NATO military alliance of building up offensive capabilities near its borders.
US, NATO and Ukrainian officials say Russia has built up forces near Ukraine, sparking fears of a looming attack.
Moscow denies any such plan. Belarus is itself locked in a row with the European Union over migrants camped at its western border.
Casting it as a response to new military deployments in countries to the west and south of Belarus, Defence Minister Viktor Khrenin said Minsk would hold an exercise with Russia in the “medium-term”.
He gave no specific date.
He said: “We see troop formations around our state borders… We can only be concerned by the militarisation of our neighbouring countries, which is why are forced to plan measures in response.”
NATO member Lithuania, which lies to the west of Belarus, said on Sunday the Atlantic alliance needed to adjust its stance towards Belarus, whose military, it said, was becoming more integrated with Russia’s armed forces.
On Monday, Belarusian leader Alexander Lukashenko said Minsk would not sit idly on the sidelines if the simmering conflict in eastern Ukraine erupted or a war broke out with the West at Russia’s borders.
In a clear nod to Russia, he said: “It is clear whose side Belarus will be on.”
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Russia’s financial and political backing helped him weather huge protests against his rule that broke out last autumn.
Lukashenko was quoted as saying by Interfax news agency: “They understand this, that’s why they’ve begun strengthening their northern Belarus-Ukraine border.”
US, NATO and Ukrainian officials warned on Tuesday that Russia would pay a high price for any new military aggression as the Western military alliance met to discuss Moscow’s intentions for massing troops on the border of the former Soviet republic.
The West has already shown that it can wield economic, financial and political sanctions against Moscow, NATO Secretary-General Jens Stoltenberg told reporters ahead of talks of the alliance’s foreign ministers in the Latvian capital Riga.
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Mr Stoltenberg said: “There will be a high price to pay for Russia if they once again use force against the independence of the nation, Ukraine.”
US Secretary of State Antony Blinken is expected to brief his 29 NATO counterparts on Washington’s intelligence on what is going on at the alliance’s eastern flank and in Ukraine, which is not a member.
He said at a news conference before the meeting: “Any escalatory actions by Russia would be a great concern to the United States… and any renewed aggression would trigger serious consequences.
“We will be consulting closely with NATO allies and partners in the days ahead… about whether there are other steps that we should take as an alliance to strengthen our defences, strengthen our resilience, strengthen our capacity.”
Kyiv’s aspirations for integration with the West have triggered a standoff with Moscow.
Lukashenko’s comments appeared to contrast with the more neutral stance taken by Lukashenko after Russia’s 2014 annexation of Crimea from Ukraine and its backing for separatist forces in Ukraine’s east.
Minsk, like most of the world, still recognises Crimea as Ukrainian territory.
The Kremlin annexed the Black Sea peninsula of Crimea from Ukraine in 2014 and then backed rebels fighting government troops in the east of the country.
That conflict has killed 14,000 people, according to Kyiv, and is still simmering.