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.
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.
“It is necessary to make the Ukrainian authorities start substantial talks [with the rebels],” he said.
Nearly 2,600 people have been killed since April, the UN says, when Russia’s annexation of Crimea prompted the rebels to take control of large parts of Donetsk and Luhansk regions in the far east of Ukraine.
Earlier, Ukrainian Prime Minister Arseny Yatsenyuk said the government was re-opening the question of Nato membership.
He said it was sending a bill to parliament calling for Ukraine’s non-aligned status to be cancelled, in effect paving the way for Ukraine to join Nato.
Mr Rasmussen indicated Nato was open to considering Ukraine’s application if it met the conditions.