The U.S. Defense Department says Russian fighter jets violated Ukrainian airspace several times in the last 24 hours.
Speaking to journalists in Washington on April 25, a Pentagon spokesman said it was not clear what the intent of the flights was but the Russian aircraft may have been testing Ukrainian tracking capabilities.
The Pentagon confirmed that U.S. Army General Martin Dempsey, chairman of the Joint Chiefs of Staff, spoke on April 24 by telephone with his Russian counterpart, General Valery Gerasimov.
Details of that conversation were not disclosed.
The United States has informed Russia that U.S. Defense Secretary Chuck Hagel would like to speak with Russian Defense Minister Sergei Shoigu, but no response has been received.
Russia is currently conducting military exercises along its border with Ukraine, a gesture Hagel has described as "dangerously destabilizing."
Based on reporting by AP and Reuters