Russian aircraft have entered Ukrainian airspace several times in the past 24 hours, according to the US.
The infringement of Ukranian airspace is the latest sign of rapidly escalating tensions between Russia and Ukraine.
The claim came from Pentagon spokesman Colonel Steven Warren, who said: "I can confirm that on several occasions in the last 24 hours, Russian aircraft have entered Ukrainian airspace."
The incidents occurred near the border with Russia amid rising tension in the east of the country.
Ukrainian Prime Minister Arseniy Yatsenyuk has accused Moscow of wanting to start "a third world war".
In a statement from the Pentagon, Col Steven Warren repeated US calls to take "immediate steps to de-escalate the situation".
He said the US had told Russian officials that US Secretary of Defense Chuck Hagel wanted to speak to his Russian counterpart Sergei Shoigu, but there had been no response so far.
Mr Hagel described Russian activity along the Ukrainian border as "dangerously destabilising" and "very provocative".
Russian foreign minister Sergei Lavrov has warned Kiev it will face justice for a "bloody crime" in eastern Ukraine, where the government has been acting against rebels.
The news of the Russian encroachment on Ukranian airspace comes as the Kiev government reported that seven members of an observer mission in the country had been seized by separatists in the flashpoint town of Slavyansk.
The US President Barack Obama and European Union leaders are expected to impose more sanctions on Russia on after failures to implement an agreement to defuse the crisis in Ukraine.
Russia in turn has accused the West of wanting to "seize" Ukraine.
On April 23 , two Russian bombers flew close to UK airspace and were chased away by an RAF jet fighter.