NATO Friday accused Russia of direct involvement in the Ukraine crisis, saying Russian military was supporting rebels in east Ukraine in a "blatant violation" of that country's sovereignty, a media report said.
After a meeting of NATO ambassadors to discuss the crisis, NATO Secretary General Anders Fogh Rasmussen said Russia's denials were "hollow", BBC reported.
The 28-member military alliance Thursday released satellite images that showed Russian forces inside Ukraine.
Russia has denied all accusations of sending troops to conflict-hit eastern Ukraine. President Vladimir Putin has blamed the Ukrainian government for the crisis.
Earlier, Ukrainian Prime Minister Arseny Yatsenyuk said the government 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.
Rasmussen indicated that the military alliance was open to considering Ukraine's application if it met the required conditions, according to media reports.
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Heavy fighting was continuing Friday near the strategic port of Mariupol, on the Azov Sea.
Rebel forces were trying to capture the city but were failing due to the unrelenting defence put up by the Ukrainian government troops.
The advance has raised fears that the Kremlin might seek to create a humanitarian corridor between Russia and Crimea -- a territory annexed by Russia from Ukraine in March.
Rebels were also reported to have surrounded government soldiers in several places near the city of Donetsk.
Ukrainian forces near the town of Ilovaysk said they were cut off and have been urgently asking for supplies and reinforcements.
However following an intervention by Moscow, rebels have agreed to give a "humanitarian corridor" to the government troops, according to Al Jazeera.
According to a UN report, at least 2,593 people have been killed and 5,043 injured in the Ukraine conflict between April and August.
The report said that 155,800 people have fled elsewhere in Ukraine while at least 188,000 have gone to Russia.