NEW : NATO chief : It 's `` the gravest crisis to European security '' since the Cold War . `` Armed opposition groups must stop all illegal actions , including detaining people and seizing public buildings in violation of Ukraine 's laws and constitution , '' she said . Russian security services say dozens militants were killed in recent days . Russian Foreign Minister Lavrov calls timing of presidential vote unusual . Lavrov rules out first international meeting on Ukrainian .
NEW : NATO chief : It 's `` the gravest crisis to European security '' since the Cold War . Russian security services say dozens militants were killed in recent days . Russian Foreign Minister Lavrov calls timing of presidential vote unusual . Lavrov rules out first international meeting on Ukrainian .