NATO foreign ministers will meet Tuesday to debate how to fight Russia's military buildup along Ukraine's border amid fears the Kremlin is preparing to invade.
The long-awaited conference in Latvia's capital Riga comes at a critical time on NATO's eastern border, as partners also cope with a migrant crisis that the West accuses Russia-backed Belarus of fueling.
Western countries, led by the US, are concerned that Moscow is plotting an assault into Ukraine after accusing Moscow of massing soldiers along the frontier.
"There is not clarity about the Russian intentions but there is an unusual concentration of forces for the second time this year," NATO Secretary-General Jens Stoltenberg told AFP during a visit to alliance forces in Latvia.
"We see heavy armour, drones, electronic warfare systems and tens of thousands of combat-ready troops."
Moscow, which annexed Crimea from Ukraine in 2014 and supports rebels fighting Kyiv, has categorically denied organizing an assault, blaming NATO for inflaming tensions.
NATO officials said the alliance is still unsure of Russian President Vladimir Putin's intentions – but ministers will discuss contingency measures in the event of an invasion.
The US-led alliance attempts to demonstrate to the Kremlin that it will face harsh consequences if it threatens Ukraine while avoiding provocation.
Officials anticipate discussions about bolstering Ukraine's military and possibly boosting NATO forces arrayed along the country's eastern flank.
However, they note that NATO-aspirant Ukraine – represented at the two-day meeting by its foreign minister – is not protected by the alliance's collective defense accord.
"We want to leave no question in people's minds that there will be severe consequences, strategic consequences for Russia, if it pursues the kind of path that we fear it may be on," a senior US official said.
"It's a question of finding the right signals and the right deterrent posture that in fact leads to a de-escalation rather than an escalation."
Friday, US Vice President Joe Biden said that he was likely to meet with Russia or Ukraine's authorities to ease escalating tensions.
'Hybrid assault'
The mounting anxieties around Ukraine come as NATO allies Poland, Lithuania, and Latvia have been confronted with another threat from the east, which will be a significant topic of discussion in Riga.
They accuse Moscow's ally Belarus of funneling thousands of migrants from the Middle East to their borders in a "hybrid attack" in retaliation for Minsk's EU sanctions.
Alexander Lukashenko, President of Belarus, denies the claim.
NATO has voiced "solidarity" with its eastern allies but has mainly remained silent as the threat level hovers short of natural aggression.
In a meeting with Stoltenberg last week, Poland's President Andrzej Duda suggested expanding the number of NATO forces positioned on the country's eastern flanks.
However, an attempt to initiate emergency talks under article 4 of the alliance's founding treaty appears to have been halted for the time being.
Stoltenberg and EU leader Ursula von der Leyen committed to intensifying cooperation against such threats during a joint Baltics tour on Sunday.
Although tensions at the border have lessened significantly as some refugees return to Iraq, Warsaw and Vilnius think 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.
