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By Aleksandar Vasovic and Alexei Anishchuk SLAVIANSK, Ukraine/ST PETERSBURG, Russia (Reuters) - Ukrainian forces killed up to five pro-Moscow rebels on Thursday as they closed in on the separatists' military stronghold in the east, and Russia launched army drills near the border in response, raising fears its troops would invade. The Ukrainian offensive amounts to the first time Kiev's troops have used lethal force to recapture territory from the fighters, who have seized swathes of eastern Ukraine since April 6 and proclaimed an independent "People's Republic of Donetsk". Ukraine's acting president accused Moscow of supporting "terrorism at the state level" against his country for backing the rebels, who the government blames for kidnapping and torturing a politician found dead on Saturday. The Ukrainian Interior Ministry said its forces backed by the army had removed three checkpoints manned by armed groups in the separatist-controlled city of Slaviansk.