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.

Ukraine's defence ministry and state security service said the troops were killed and seven others wounded when their armoured 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 20 kilometres from Kramatorsk, the ministry said in a statement on its website.

"In all, as a result of the prolonged fighting, six members of the armed forces were killed. Eight soldiers were wounded, one of them seriously," it said.

The state security service (SBU) said later that the seriously wounded soldier had died while being taken to hospital.

Rebel leaders held referendums in two eastern regions on Sunday which they said 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 incident, defence minister Mykhailo Koval said a total of nine servicemen 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 who died when their helicopters were downed by separatist fire, Mr Koval said.

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 Yanukovych in February.

Russia throws future of International Space Station into doubt

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 which was supposed to end the space race of the Cold War era.

Deputy prime minister Dmitry Rogozin said Moscow would reject a US request to prolong the orbiting station's use beyond 2020. It would 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 politicises everything," Mr Rogozin told a news conference.

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

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

Moscow's response would affect NK-33 and RD-180 rocket engines which Russia supplies to the United States.

"We are ready to deliver these engines but on one condition: that they will not be used to launch military satellites," Mr Rogozin said.

RD-180 engines are used to boost Atlas 5 rockets made by United Launch Alliance, a partnership of Lockheed Martin and Boeing that holds a virtual monopoly on launching US military satellites.

Mr Rogozin said Russia would also suspend operation of GPS satellite navigation system sites on its territory from June, unless an agreement could be reached by September under which similar sites could be opened in the US for Russia's own system, Glonass.

Kiev's handling of the crisis in the east caused angry scenes in the Ukrainian parliament on Tuesday when deputies of the formerly ruling Regions party and their communist allies attacked the government for sending in the army - a move they said had alienated Russian-speakers.

"The anti-terrorist operation has turned into a terrorist operation, organised by the authorities against their own people," Regions deputy Mykola Levchenko said.

When communist faction leader Petro Symonenko took a similar line, acting president Oleksander Turchinov, who is also speaker, burst out: "Liars have no place in parliament. Sit down."

Reuters