"Nato has accused Russia of a "blatant violation" of Ukraine's sovereignty, saying it is engaged in direct military operations to support rebels.
Secretary-General Anders Fogh Rasmussen said that "despite hollow denials", it was now clear that Russia had illegally crossed Ukraine's border.
He said Nato would respect any Ukrainian decision on security, after its PM said he was putting the country on course for Nato membership.
Russia denies sending troops and arms.
Russian President Vladimir Putin blamed the Ukrainian government for the crisis, comparing its siege of two cities held by separatists, Donetsk and Luhansk, to the siege of Leningrad by the Nazis in World War Two."
"Speaking at a news conference after a meeting of Nato ambassadors to discuss the crisis, Mr Rasmussen said that it was clear Russia had illegally crossed the border into Ukraine and was engaged in direct military operations there.
"This is not an isolated action, but part of a dangerous pattern over many months to destabilise Ukraine as a sovereign nation," he said.
"This is a blatant violation of Ukraine's sovereignty and territorial integrity. It defies all diplomatic efforts for a peaceful solution."
He called on Russia to cease military action, stop supporting the rebels and "take immediate and verifiable steps towards de-escalation of this grave crisis".
Mr Rasmussen indicated Nato was open to considering Ukraine's application to join if it met the conditions."
BBC