NATO has been discussing how react to a military build-up by Moscow close to Ukraine's border amid fears from Western nations that Russian President Vladimir Putin could be planning an incursion.

Ukrainian foreign minister Dmytro Kuleba called for a three-pronged approach involving clear communications to Moscow, readying a package of sanctions and increasing military support to Kiev.

"We are confident that if we join efforts, if we act in a coordinated fashion, we will be able to deter President Putin and to demotivate him from choosing the worst-case scenario, which is a military operation," he said as he joined the meeting of NATO foreign ministers in Riga.

NATO chief Jens Stoltenberg warned after talks among allies on Tuesday that Russia would face a "high price" if it launches an operation against Ukraine and said alliance members could impose sanctions.

NATO itself does not decide on sanctions.

Ukraine, which has seen its push to join the US-led bloc stall, has been receiving increased military support from NATO allies.

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

Russian President Vladimir Putin said Tuesday that military exercises and other moves by the West and Ukraine threaten Russia's security, warning against crossing the Kremlin's "red lines".

"Look, they spoke about a possible Russian military intervention in Ukraine at the beginning of the year. But as you see this did not happen," Putin said.

The new build-up follows a similar surge in the spring, when Russia gathered around 100,000 troops on Ukraine's borders but later announced a drawdown.