The US President Barack Obama is accusing Russia of sending forces across the border into the east of the country.
NATO has released satellite images showing at least 1,000 troops crossing into Ukraine.
However the Russian Foreign Minister Sergei Lavrov is rejecting the allegations – describing the claims as conjecture.
NATO Secretary-General Anders Fogh Rasmussen is condemning what he says is Russian disregard of international obligations.
“We urge Russia to cease its illegal military actions, stop its support to armed separatists, and take immediate and verifiable steps towards de-escalation of this grave crisis” he said.
