Russia has “combat-ready troops” on the border with Ukraine as part of an unexplained military build-up, the secretary general of Nato said on Friday.
Jens Stoltenberg said Moscow’s intentions were unclear but said the build-up was concerning because Russia has flouted Ukraine’s borders before.
He told the Kremlin that any use of force against Ukraine “will have consequences”.
Ukrainian President Volodymyr Zelensky alleged on Friday that intelligence services had uncovered plans for a Russian-backed coup in Kiev.
He claimed there were audio recordings of Russian and Ukrainian officials discussing a plot that would drag in a prominent billionaire.
If Russia invades, the Ukrainian army is “entirely prepared”, he said. Moscow denies either plotting a coup or planning to invade neighbouring Ukraine.
“Russia had no plans to get involved,” said Kremlin spokesman Dmitry Peskov. “Russia never does such things at all.”
Nato foreign ministers will meet in Riga, Latvia, next week with the threat from Russia high on the agenda.
Poland, a Nato member, suspects Russia of being behind the border crisis in Belarus, where thousands of migrants have tried to breach the EU’s eastern border.
Mr Stoltenberg said Russia’s record of hybrid attacks, cyber warfare and aggressive rhetoric all raised concerns about its intentions in Ukraine.
Although Ukraine is not part of Nato and its mutual defence guarantee, the alliance “is there to defend and protect all allies if that is needed”, Mr Stoltenberg said.
“This is the second time this year that Russia has amassed a large and unusual concentration of forces in the region,” he said.
“This includes heavy capabilities like tanks, artillery, armoured units, drones and electronic warfare systems, as well as combat-ready troops.”
He called on Russia to be transparent about its military build-up and to de-escalate tension to prevent miscalculations.
“This is a military build-up by a country which has invaded Ukraine before, that did so in 2014 when they illegally annexed Crimea,” Mr Stoltenberg said.
“Any use of force against Ukraine will have consequences, will have costs.”
Russia annexed Crimea in 2014, a move regarded as illegal by western countries. It was subsequently hit with sanctions and expelled from what is now the G7.
Separatists loyal to Russia have fought Ukraine’s government in the eastern Donbas region in a years-long conflict.
Angela Merkel, Germany’s departing chancellor, criticised Russia on Thursday for shunning talks to reduce tension in Donbas.
“It would have been a good sign that all sides are interested in a solution … but it didn’t come to that” before she leaves office, she said.
