U.S. Says Russian Aircraft Violated Ukraine's Airspace

Russian warplanes violated Ukraine's airspace several times in the past 24 hours, a Pentagon spokesman said Friday, 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.

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, U.S. Defense Secretary Chuck Hagel had tried to arrange a phone conversation since Thursday with his Russian counterpart, Sergei Shoigu, to discuss the crisis, but Moscow has yet to respond to the request, Warren said.

The chairman of the U.S. Joint Chiefs of Staff, General Martin Dempsey, had spoken to his military counterpart in Moscow, Warren said.

After Ukraine announced military operations to counter pro-Kremlin rebels, Russia ordered its troops and aircraft massed on the border to launch new exercises. 

U.S. defense officials have confirmed a flurry of troop movements associated with the declared drills and acknowledged there is ample Russian firepower on the border to quickly seize territory in eastern Ukraine -- if Moscow chose to act.

Germany meanwhile said pro-Russian separatists had detained 13 members of an OSCE observer mission in Ukraine.

Kiev's deputy foreign minister, Danylo Lubkivsky, said at the United Nations on Friday that his government would exercise restraint its moves against separatists.

But if Russia crossed the border, "we will fight to defend our country," he said.