Nato Secretary-General Anders Fogh Rasmussen said the alliance had seen what he called "a Russian incursion into Ukraine."
Responding to reports that a Russian column had entered Ukraine overnight, the Nato chief said: "It just confirms the fact that we see a continuous flow of weapons and fighters from Russia into eastern Ukraine and it is a clear demonstration of continued Russian involvement in the destabilisation of eastern Ukraine."
Mr Rasmussen also said Europe is becoming more dangerous and less stable.
More top news
Awarded the Victoria Cross for rescuing colleagues from the Somme, a Northern Irish soldier has been honoured with a statue in his town.
China has become Africa's biggest trading partner - but amid accusations of exploitation, all is not well in this remarkable partnership.
A newborn baby girl has been killed by dogs after they attacked her while she was lying on a sofa.