NATO foreign ministers will meet on Tuesday to examine President Vladimir Putin’s probable plan to concentrate Russian soldiers on Ukraine’s borders, as well as the alliance’s response, after Belarus announced joint military exercises with Russia.

During the conference in Latvia’s capital, Riga, US Secretary of State Antony Blinken is due to inform his 29 NATO colleagues on the US intelligence picture on the alliance’s eastern flank and in Ukraine, which is not a NATO member.

“We don’t know what Russia’s objectives are, but we do know that there is an extraordinary concentration of Russian military troops near Ukraine,” NATO Secretary-General Jens Stoltenberg said on Monday.

“We see heavy capabilities, we see armored units, drones, electronic warfare systems and we see tens of thousands combat-ready Russian troops,” he said after visiting NATO troops rehearsing battle skills with camouflaged tanks and live rounds in a snowy woodland north of Riga.

The Western military alliance is alarmed by the Russian military presence on Ukraine’s borders, the second build-up this year. In May, Russian troops numbered 100,000 on the Ukraine border, the largest number since Russia’s annexation of Crimea in 2014, Western officials say.

Belarus on Monday announced joint military drills with Russia on its border with Ukraine and accused NATO of building up offensive capabilities near its borders.

President Alexander Lukashenko, who the West accuses of seeking to divide the European Union by sending Middle Eastern migrants to the Polish border, warned Minsk would not sit on the sidelines in the case of all-out war.

“It is clear whose side Belarus will be on,” he said, referring to Russia, whose financial and political backing helped him weather huge protests following disputed presidential elections in August last year.