NEW : NATO chief : It 's `` the gravest crisis to European security '' since the Cold War . " Russian Foreign Minister Sergey Lavrov calls timing of presidential vote unusual . Lavrov rules out second international meeting on Ukraine .
NEW : NATO chief : It 's `` the gravest crisis to European security '' since the Cold War . Ukrainian security services say 30 militants were killed in recent days . Russian Foreign Minister Sergey Lavrov calls timing of presidential vote unusual . Lavrov rules out second international meeting on Ukraine .