U.S. military officials said Friday that Russian aircraft flew into Ukrainian airspace several times in the previous 24 hours.
A Pentagon spokesman called on Moscow "to take immediate steps to de-escalate the situation." The official gave no details on where the incursions took place or the goal of the action.
The flights come as Russia increases military exercises along the Ukrainian border. On Thursday, the top U.S. military officer spoke with his Russian counterpart.
Also Friday, pro-Russian separatists in eastern Ukraine detained a team of international military observers. The separatists seized a bus carrying at least 13 people from the Vienna-based Organization for the Security and Cooperation in Europe near the town of Slovyansk.
The town's self-declared mayor said the group was detained because a Ukrainian military official was traveling with them.
"The sign OSCE does not mean protection for an officer of the (army) General Headquarters. We found an employee of the army headquarters. After an investigation we will decide what we are to do."
The OSCE wrote on Twitter that it had lost contact with the German-led monitoring team.
A U.S. State Department spokeswoman condemned the detentions, calling the tactic repressive and cowardly.
"We are deeply concerned about reports that unidentified gunmen have abducted a Vienna document inspection team - that's of course part of the OSCE - and their Ukrainian escorts in the town of Slovyansk. The team was reportedly taken to the administrative building which is being held by armed pro-Russian separatists."
Pro-Russian militants are occupying government buildings in around a dozen cities in eastern Ukraine.
Ukrainian Prime Minister Arseniy Yatsenyuk has accused Russia of wanting to occupy Ukraine "militarily and politically," as both Kyiv and Moscow mass troops close to their mutual border. Mr. Yatsenyuk told an interim Cabinet meeting that Moscow "wants to start World War Three."
Meanwhile, U.S. President Barack Obama discussed the Ukraine crisis in a telephone call with French President François Hollande, German Chancellor Angela Merkel, Italian Prime Minister Matteo Renzi and British Prime Minister David Cameron.
According to the White House, Mr. Obama said the United States is prepared to impose targeted sanctions against Russia.
Mr. Obama has criticized what he called Russia's "further meddling" in eastern Ukraine, saying Russian President Vladimir Putin must decide whether he wants to see his country's already fragile economy weakened further because he failed to act diplomatically in Ukraine.