"We're deeply concerned by evidence that Russia has made plans for significant aggressive moves against Ukraine, plans include efforts to destabilise Ukraine from within as well as large scale military operations," Blinken said after a meeting of NATO foreign ministers in Latvia's capital Riga, accusing Moscow of massing "tens of thousands of additional combat forces" near the border.

"Now, we don't know whether President Putin has made the decision to invade. We do know that he's putting in place the capacity to do so on short order, should he so decide," he said.

Blinken insisted "diplomacy is the only responsible way to resolve this potential crisis" ahead of a meeting with Russian counterpart Sergei Lavrov in Sweden on Thursday.

But he warned that there would be "far-reaching and long-lasting consequences" for Moscow if it pushed ahead with any aggression.

"We made it clear to the Kremlin that we will respond resolutely, including with a range of high-impact economic measures that we've refrained from using in the past," Blinken said.

The top US diplomat also insisted that NATO allies were "making sure that Ukraine has the means to defend itself".

And he said that the alliance would "look at what it needs to do in the event of further Russian aggression, to shore up its own defences" on discussion of bolstering forces along NATO's eastern flank.

Moscow, which seized Crimea from Ukraine in 2014 and backs separatists fighting Kiev, has strongly denied it is plotting an attack and blames NATO for fuelling tensions.

Moscow on Wednesday accused Ukraine of massing tens of thousands of troops in the country's east, as worries grow over an escalation.

Also on Wednesday, Ukrainian President Volodymyr Zelensky called for direct talks with Moscow as tensions mounted over the claims that Russia was laying the groundwork for an invasion.