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 United States 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.

NATO diplomats say the bloc remains uncertain of Russian President Vladimir Putin's intentions -- 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 strengthening 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 called on Russia to "show transparency, reduce tensions and de-escalate."

"This military build-up is unprovoked and unexplained. It raises tensions and risks miscalculation," NATO chief Jens Stoltenberg said in Latvia Sunday.

- '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.

Speaking on a joint tour of the Baltics on Sunday, Stoltenberg and EU chief Ursula von der Leyen pledged to step up cooperation against such challenges.

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

"There can be no doubt that Lukashenko's regime and the forces that support it will continue to test the Western world's unity and their ability to react," Lithuanian President Gitanas Nauseda warned last week.