LONDON (MarketWatch) -- Russia's 45th Airborne Regiment has been spotted on the ground in the eastern Ukrainian cities of Slavyansk and Kramatorsk, Ukraine's First Deputy Prime Minister Vitali Yarema said on Channel 5 on Tuesday, according to an unconfirmed report from Bloomberg. The move comes as Ukraine earlier on Tuesday launched a military operation to regain control of cities in the country's east, where the majority of sites commandeered by pro-Russian forces are located. Russia's state-run RIA Novosti news service reported that four militants were killed and two wounded when Ukrainian forces stormed an airport in Kramatorsk, according to Bloomberg. Earlier on Tuesday, Russia repeated that any action against pro-Moscow separatists could push Ukraine closer to a civil war. European stock markets SXXP, -0.66% took a dive in afternoon trading on the rising tension on the continent's eastern borders.