U.S. President Barack Obama spoke with German Chancellor Angela Merkel by telephone Thursday about the situation in Ukraine, the White House said. White House spokesman Jay Carney said that Obama spoke to Merkel from Air Force One as he flew back to Washington. The call came amid increasing concern in Europe that the Ukraine crisis could lead to a cut-off of Russian gas. Russian President Vladimir Putin has told European leaders that Ukraine's growing debts to Moscow can threaten supplies to Europe. A large portion of Europe's gas comes from Russia through Ukraine.