Local commuters walk along railway lines next to Ukrainian tanks ready to depart from Crimea to Ukraine onboard a train in the settlement of Gvardeiskoye near the Crimean city of Simferopol March 31, 2014 (Photo: Reuters)
NATO foreign ministers will gather in Brussels on Tuesday as the defence alliance seeks to reinforce its eastern frontier against a resurgent Russia emboldened by the annexation of Crimea.
In a regular two-day meeting of the 28 ministers, including US Secretary of State John Kerry, NATO will confirm the suspension of cooperation with Moscow, a decision made on March 5 after Russian troops grabbed Crimea from Ukraine.
"Reassuring allies is most important for NATO," said Douglas Lute, the US ambassador to the Brussels-based Western alliance, in a pre-meeting briefing.
Hoping to avoid antagonising Moscow, NATO agreed in the 1990s to keep permanent troops out of new member countries that border Russia.
But after the Ukrainian crisis, NATO has not ruled out the possibility of placing permanent military bases in the Baltic countries.
"We cannot do business as usual with Russia," Lute said.
"It is clear that Russia has not played by the rules, has not been consistent with our partnership ... so we can review our own rules."
The US has been at the forefront of responding to Russian provocations in Ukraine, hoping to balance its desire to reinvigorate the NATO alliance while keeping diplomatic channels with the Kremlin open.
Over the weekend, General Philip Breedlove, NATO's supreme allied commander, was sent back to Europe early amid what the Pentagon called Russia's "lack of transparency" over the Ukraine crisis.
That announcement came as Kerry met his Russian counterpart Sergei Lavrov in Paris in the hope of easing tensions over the annexation of Crimea and the buildup of Russian forces on the border with Ukraine.
Since the Crimea crisis began, NATO has flown AWACS radar aircraft over Poland and Romania, and the US has dispatched fighter jets to Lithuania and Poland.
NATO will hold air drills over the ex-Soviet Baltic countries on Tuesday -- an annual exercise that has gained added significance amid the concerns over Russia's territorial ambitions.
Several other NATO powers, including France and Britain, have also offered additional warplanes, while Germany will allegedly follow suit according to a report from German magazine Der Spiegel.
NATO is also mulling ways to boost Kiev directly, including steps to modernise its army, but without taking on an operational role in the ex-Soviet republic.