NATO allies have rushed to bolster their eastern flank after Russian President Vladimir Putin ordered the attack, but remain adamant that they will not get involved militarily in the war in non-NATO member Ukraine.

NATO has for the first time activated its rapid response force to add to thousands of troops already sent by allies to eastern members.

The alliance has faced calls to help impose a no-fly zone over Ukraine as Russia has launched air strikes against civilian targets.

NATO has rejected that idea, arguing it could drag the alliance what could turn into a nuclear conflict with with Moscow.

"We have to accept the reality that that involves shooting down Russian planes," British Prime Minister Boris Johnson said Tuesday on a trip to Estonia with NATO chief Jens Stoltenberg.

"That's a very, very big step that is simply not on the agenda of any NATO country."