NATO accused Russia on Friady, Aug 29, of a "blatant violation" of Ukraine's sovereignty and engaging in direct military operations to support pro-Russian fighters, BBC News reported.

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 pro-Moscow militias, 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.

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.

Speaking at a news conference after an emergency meeting of NATO ambassadors, Rasmussen said that it was clear Russia was engaged in direct military operations in Ukraine.

"This is not an isolated action, but part of a dangerous pattern over many months to destabilize 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."

Rasmussen indicated NATO was open to considering Ukraine's application to join if it met the conditions.

In 2008, Ukraine applied for NATO membership under then-President Viktor Yushchenko. But the plans were shelved two years later when Viktor Yanukovych came to power and ended Ukraine's NATO ambitions in favor of mending relations with Russia.

Rasmussen said on Friday: "I am not going to interfere with political discussions in Ukraine, but let me remind you of NATO's decision taken at the Bucharest Summit in 2008 according to which Ukraine will become a member of NATO, provided of course that Ukraine so wishes and provided that Ukraine fulfils the necessary criteria."

On Thursday, NATO released satellite images it said showed columns of Russian armed forces inside Ukrainian territory, adding that more than 1,000 Russian troops were operating inside Ukraine.