Trucks drivers from a Russian aid convoy gather outside Kamensk-Shakhtinsky in the Rostov region, near the Russian-Ukrainian border.

NATO Secretary General Anders Fogh Rasmussen said on Friday a "Russian incursion" into Ukraine had occurred overnight, but stopped short of characterizing it as an invasion.

"Last night we saw a Russian incursion, a crossing of the Ukrainian border," he told reporters after meeting the Danish defense minister.