By Thomas Grove and Gabriela Baczynska SLAVIANSK/DONETSK, Ukraine (Reuters) - Pro-Russian separatists on Monday ignored an ultimatum to leave occupied government buildings in eastern Ukraine and instead seized more buildings as the government failed to follow through on a threatened military crackdown. U.S. President Barack Obama criticized Russia in a telephone conversation with Russian President Vladimir Putin later on Monday, saying Moscow's actions in Ukraine were not conducive to a diplomatic solution. The Kremlin said it had requested the call. The White House said the call was frank and direct.