Russian warplanes have violated Ukrainian airspace several times, according to the US Defense Department. The alleged incursions come as Moscow has conducted military exercises near Ukraine's eastern border.
The Russian warplanes may have been testing Ukraine's radar systems or simply making a show of force, according to US officials on Friday.
"I can confirm that on several occasions in the last 24 hours, Russian aircraft have entered Ukrainian airspace," said Colonel Steven Warren, who urged Moscow to take "immediate steps to de-escalate the situation."
Military tensions have risen precipitously this week, after Ukrainian forces clashed with pro-Russian separatists in the eastern city of Slovyansk on Thursday. Five separatists died in the clash. Kyiv has said its forces would blockade the city to cut off the separatists from any reinforcements.
Russian President Vladimir Putin (in photo) warned that the operation in Slovyansk would have consequences for Kyiv.
"It is a punitive operation and it will of course incur consequences for the people making these decisions, including on our interstate relations," Putin said during a televised meeting.
Russian military maneuvers
The Kremlin has responded to the clash by conducting renewed military exercises near Ukraine's eastern border. According to Kyiv, Russian troops have come within one kilometer of the two countries' frontier.
Moscow has massed some 40,000 troops near the Ukrainian border, according to NATO estimates. President Putin has claimed the right to intervene militarily on behalf of Russian speakers if their rights are violated.
Speaking at the United Nations on Friday, Ukraine's deputy foreign minister told reporters that he feared a Russian invasion.
"We have information that we are in danger," Danylo Lubkivsky said, calling the Russian military maneuvers a "very dangerous development."
"We are going to protect our motherland against any invasion," Lubkivsky added. "We call on the Russians to stop this madness."
slk/jm (AP, AFP, dpa, Reuters)