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.

About 30 rebels, who had taken cover among bushes along a river, attacked with grenade-launchers and automatic weapons near a village 12 miles from Kramatorsk, the ministry said on its website.

Rebel leaders held referendums in two eastern regions on Sunday and backed self-rule overwhelmingly. While Kiev and the West denounced the votes as illegal, the rebels called on Monday for their regions to become part of Russia. Moscow has stopped short of endorsing their bid for annexation.

Before the Kramatorsk attack, Defense Minister Mykhailo Koval said a total of nine troops had been killed so far in the army's "anti-terrorist" operation, which has been directed mainly against rebels in the towns of Slaviansk and Mariupol.

The dead included five air crew, Koval said. They died when their helicopters were downed by separatist fire.

Rebels have also suffered losses in the uprising, which began with the seizure of public buildings in eastern towns and cities. Many of the separatists hope to follow Crimea, which voted for union with Russia before its formal annexation in March.

The United States says Russia is backing the rebels while the Kremlin accuses Washington of having helped protesters to topple pro-Moscow Ukrainian President Viktor Yanukovich in February.

In the worst East-West crisis since the 1991 collapse of the Soviet Union, Washington and the European Union have slapped sanctions on a limited number of Russians and pro-Russian Ukrainians, and some small firms. Washington has also said it would deny export licences for high-technology items that could help the Russian military.

Moscow retaliated on Tuesday, casting doubt on the long-term future of the International Space Station, a $100 billion, 15-nation project that was supposed to end the space race of the Cold War era.

Deputy Prime Minister Dmitry Rogozin said Moscow will reject a U.S. request to prolong the orbiting station's use beyond 2020. It will also bar Washington from using Russian-made rocket engines to launch military satellites.

"We are very concerned about continuing to develop high-tech projects with such an unreliable partner as the United States, which politicizes everything," Rogozin told a news conference.

Washington wants to keep space station in use until at least 2024. But since the end of the U.S. Space Shuttle program, Russia's Soyuz spacecraft have been the only way to get there.

The U.S. space agency NASA is working with companies to develop space taxis with the goal of restoring U.S. transport to the station by 2017. The United States currently pays Russia more than $60 million per person to fly its astronauts up.