Russian warplanes have violated Ukraine's airspace several times in a 24-hour period, a Pentagon spokesman says, in the latest sign of a mounting confrontation between Moscow and Kiev.
"I can confirm that on several occasions in the last 24 hours, Russian aircraft have entered Ukrainian airspace," said Colonel Steven Warren on Friday.
He did not provide more details, including where the incidents occurred or what kind of Russian planes were involved.
Warren urged Russia to take "immediate steps to de-escalate the situation."
With tensions soaring, US Defence Secretary Chuck Hagel had tried to arrange a phone conversation with his Russian counterpart, Sergei Shoigu, to discuss the crisis, but Moscow has yet to respond to the request, Warren said.
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The chairman of the US Joint Chiefs of Staff, General Martin Dempsey, has spoken to his military counterpart in Moscow, Warren said.
After Ukraine announced military operations to counter pro-Kremlin rebels, Russia ordered its troops massed on the border to launch new exercises.
