NEW : NATO chief : It 's ` ` the gravest threat to European security '' since the Cold War . Ukrainian security services say 30 militants were killed in recent days . Russian Foreign Minister Sergey ##ev says timing of presidential vote unusual . His words came as violence escalated , with Ukraine 's military seeking to regain control of swaths of the country that pro-Russian militia groups have seized . Lavrov rules out second international meeting on Ukraine .
NEW : NATO chief : It 's ` ` the gravest threat to European security '' since the Cold War . Ukrainian security services say 30 militants were killed in recent days . Russian Foreign Minister Sergey ##ev says timing of presidential vote unusual . Lavrov rules out second international meeting on Ukraine .