NATO has seen an increase in Russian troops and equipment along the Ukraine border and is looking into reports of Russian tanks crossing the border into eastern Ukraine, a NATO military officer said on Friday.

"We are aware of the reports of Russian troops and tanks crossing the border between Ukraine and Russia and are looking into these reports. We can confirm a recent increase in Russian troops and equipment along the eastern border of Ukraine," the officer said, on condition of anonymity.

"If this crossing into Ukraine is confirmed it would be further evidence of Russia's aggression and direct involvement in destabilizing Ukraine," he said, after the Kiev military said a column of tanks, howitzer artillery systems and trucks had crossed into eastern Ukraine from Russia.