“We have to be clear-eyed, we need to be realistic about the challenges we face. And what we see is a significant, large Russian military build-up,” Stoltenberg told a news conference with Ukraine’s Foreign Minister Dmytro Kuleba in Brussels.

He said he did not want to speculate about Russia’s intentions but added: “We see an unusual concentration of troops, and we know that Russia has been willing to use these types of military capabilities before to conduct aggressive actions against Ukraine.”

The Russian troop movements near the Ukrainian border have over the past days spurred fears of a possible attack. Moscow has dismissed such suggestions as inflammatory and complained about increasing activity in the region by the NATO transatlantic alliance.

Russia annexed Crimea from Ukraine in 2014 and says the waters around it belong to Moscow now, despite most countries continuing to recognise the peninsula as Ukrainian.

Russian-backed separatists took control of Ukraine’s eastern Donbass region that same year and soldiers on both sides continue to be regularly killed in the conflict there.

Ukrainian President Volodymyr Zelenskiy said last week that there were nearly 100,000 Russian soldiers near Ukraine’s border.

Stoltenberg said the build-up near the border was dangerous because it reduced the amount of warning time, should Russia decide to “conduct a military aggressive action against Ukraine.” Kuleba also said he was worried that Russia could quickly activate massed troops at the border.

“It can go either way,” Lithuania’s foreign minister Gabrielius Landsbergis told reporters, also on Monday, on the margins of a meeting with his European Union counterparts.

The West cannot exclude a Russian attack on Ukraine while international attention is focused on the Belarus migration crisis, or that Russia establishes a permanent military presence in Belarus, he said.

“I would not exclude that as a possibility,” he said.

By Sabine Siebold and Robin Emmott.

(Reporting by Sabine Siebold and Robin Emmott; Writing by Ingrid Melander; Editing by Alex Richardson, William Maclean).