The long-scheduled meeting of NATO top diplomats in Latvia's capital Riga comes at a volatile moment along the bloc's eastern flank as allies also grapple with a migrant crisis the West says is fuelled by Kremlin-backed Belarus.

Western countries spearheaded by the US worry that Moscow could be planning an incursion into Ukraine after accusing the Kremlin of massing tens of thousands of troops and heavy armour close to the frontier.

"Any escalatory actions by Russia would be of great concern to the United States... and any renewed aggression would trigger serious consequences," US Secretary of State Antony Blinken told journalists ahead of the meeting.

"We have seen Russia's playbook many times over."

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.

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.

NATO diplomats say the bloc remains uncertain of Putin's intentions this time round -- but ministers will discuss contingency plans should Russia invade.

The US-led alliance is looking to show the Kremlin it faces severe costs if it threatens Ukraine, while stopping short of provoking Moscow into further aggression.

Officials expect talks on additional support for Ukraine's military and potentially a move to strengthen NATO forces arrayed along its eastern wing.

But they point out that NATO-aspirant Ukraine -- which will have its foreign minister arriving for day two of the meeting on Wednesday -- is not covered by the alliance's collective defence pact.

NATO Secretary General Jens Stoltenberg said alliance members had "different options" to respond if Russia moved on Ukraine.

"We have demonstrated over the years in reaction to Russia's previous use of military force against Ukraine that we can sustain heavy economic and financial sanctions, political sanctions," he said.

- 'Hybrid attack' -

The growing fears around Ukraine come as NATO members Poland, Lithuania and Latvia have faced another threat coming from the east that will be high on the agenda in Riga.

They accuse Moscow's ally Belarus of funnelling thousands of mainly Middle Eastern migrants to their borders in a "hybrid attack" as retribution for EU sanctions against Minsk.

President Alexander Lukashenko rejects the claim.

NATO has expressed "solidarity" with its eastern members, but has largely been left on the sidelines as the threat level floats in a grey zone just short of actual aggression.

Poland's President Andrzej Duda mooted increasing NATO force numbers deployed on its eastern flanks at a meeting with Stoltenberg last week.

But a move to trigger emergency consultations under article 4 of the alliance's founding treaty appears to have been put on hold for now.

Border tensions have eased slightly as some migrants have begun returning to Iraq, but Warsaw and Vilnius insist the crisis is far from over.

The European Union, US and other NATO members are set to hit Belarus with a fresh raft of sanctions in the coming days.

"Lukashenko should stop using vulnerable people as pawns in a political game against other countries," Stoltenberg said.

"We work very closely with the European Union, because neither the European Union or NATO has all the tools in the toolbox. But together, we can provide a strong response to what we see."