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
James Deen, one of the world's most famous porn stars, denies allegations of sexual abuse made by several women, including former partners.
The Good Morning Britain team have been showing support for Text Santa by performing their own version of the Wham! hit Last Christmas.
A seven-month-old baby was shot and killed when a shotgun belonging to his mother's boyfriend went off in their living room, police say.