Pro-Russian separatists ambushed Ukrainian troops on Tuesday, killing seven, in the heaviest loss of life for government forces in a single clash since Kiev sent soldiers to put down a rebellion in the country's east.

With the uprising and Russia's annexation of Crimea poisoning East-West relations, Moscow retaliated against U.S. sanctions by hitting aerospace projects, including refusing to extend the life of the International Space Station, a showcase of post-Cold War cooperation.

In Kiev, Ukraine's defense ministry and state security service said the troops were killed and seven others wounded when their armored column was ambushed near the town of Kramatorsk, one of several hot spots in the largely Russian-speaking east where the army has had scant success against the rebels.