Nato has accused Russia of a "blatant violation" of Ukraine's sovereignty and engaging in direct military operations to support pro-Russian rebels, reports GHN based on BBC.
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 Nazi Germany 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.
Heavy fighting is continuing near Ukraine's strategic port of Mariupol, on the Azov Sea. Rebel forces are trying to capture the city but Ukrainian government troops are digging in.
Separatists are also reported to have surrounded government soldiers in several places further north, near Donetsk city.
Ukrainian forces near the town of Ilovaysk say they are cut off and have been urgently asking for supplies and reinforcements.
Earlier, Ukrainian Prime Minister Arseny Yatsenyuk said the government was reopening 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.