NATO will discuss new steps to reinforce eastern European countries worried by Russia's annexation of Crimea and on how to bolster Ukraine's armed forces.
NATO foreign ministers will look at options ranging from stepped-up military exercises and sending more forces to eastern member states, to the permanent basing of alliance forces there.
Russia would almost certainly see that as provocative.
Ministers from the 28 alliance members are meeting in Brussels for the first time since Russia's military occupation and annexation of Ukraine's Crimea region caused the worst East-West crisis since the Cold War.
While the United States and its allies have made clear they will not intervene militarily in Ukraine, which does not belong to NATO, they have scrambled to reassure anxious NATO members in eastern Europe, particularly ex-Soviet republics in the Baltics, that they are sheltered by the alliance's security umbrella.
The US has increased the number of US aircraft in regular NATO air patrols over the Baltic States and beefed up a previously planned training exercise with the Polish air force.
"We should do everything we can to reassure our friends and colleagues in Latvia, Lithuania and Estonia and in Poland that we really believe in their NATO membership and the guarantees that we have given them," British Prime MInister David Cameron said.
The Ukrainian parliament has this morning approved joint NATO exercises on its territory.
US Ambassador to NATO Douglas Lute said the ministers, including US Secretary of State John Kerry, would discuss further measures to reassure the eastern European allies.
"They will talk about ... what more can be done to amplify the measures that have been taken already and to sustain them over time so that these measures are not simply short-term gestures," Mr Lute told a news conference.
Since 1999, when it began admitting former members of the Soviet-led Warsaw Pact, NATO has had a self-imposed restriction on permanently basing alliance forces in eastern Europe.
However, Poland and Romania have agreed to host parts of the US anti-ballistic missile shield and NATO air forces take turns to provide air cover over the Baltic states.
A senior NATO diplomat said the Crimean crisis would probably lead to the issue of permanent bases being discussed.
"I think that these are the sorts of things that ministers are likely to talk about in the next couple of days," he said.
The ministers' meeting continues tomorrow, although it is due to discuss Afghanistan then.
Russia yesterday said it was pulling back some troops from near Ukraine's eastern border.
However, NATO Secretary General Anders Fogh Rasmussen said today it has had no confirmation of any troop withdrawal.
Ben Rhodes, White House deputy national security adviser, told reporters last week that the US would increase temporary deployments of ground and naval forces to NATO allies in eastern Europe.
"We expect other European partners to step up and join us in doing so," he said.
US forces in Europe have fallen from just over 300,000 in the final years of the Cold War to about 100,000 in 2005.
The number is estimated at about 80,000 in 2014, including 14,000 civilians, according to the US military's European Command.
Elsewhere, Russia's top natural gas producer, Gazprom, has said it is increasing the gas price for Ukraine.
It will now charge $385.5 (€280) per 1,000 cubic metres in the second quarter from the previous rate of $268.5 (€195).
The decision ends a discount agreed before the political crisis.
Gazprom's Chief Executive Officer Alexei Miller said Ukraine's debt for unpaid gas bills stands at $1.7 billion (€1.23bn).