Russian planes have violated Ukrainian airspace several times over the past 24 hours, a Pentagon spokesman said Friday.
Pentagon spokesperson Colonel Steven Warren confirmed the breach on Friday, but did not provide more details, includign where the incidents occured or what kind of Russian planes were involved.
He urged Russia to take "immediate steps to de-escalate the situation," AFP news agency reported.
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.
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.
The remarks from the Pentagon came hours after armed separatists in the eastern Ukrainian city of Slovyansk reportedly seized a bus carrying international observers from the Organisation for Security and Cooperation in Europe (OSCE).
The separatist leader in Slovyansk told reporters a problem had arisen when the observers tried to pass a separatist checkpoint, and that there was an Ukrainian "spy" among the group. He did not say where they were.
The interior ministry in Kiev said the group, which included eight OSCE representatives and five members of the Ukrainian armed forces, was being held in the building of the state security agency (SBU) in the city which has been occupied by pro-Russian separatists.
"Negotiations are going on for their release," a ministry statement said.
Slovyansk, a city of around 130,000, has been for two weeks under the control of separatists who, like similar groups elsewhere in eastern Ukraine, oppose the central government in Kiev after the overthrow of a Kremlin-backed president.