KIEV, May 13 - 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, Reuters reported.

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 20 km 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, Defense Minister Mykhaylo 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, 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.

Germany's foreign minister Frank-Walter Steinmeier expressed hope on a visit to Kiev that "round table" talks between politicians and civil groups this week would help to disarm the separatists and improve the atmosphere for presidential elections due later this month.

Chancellor Angela Merkel, speaking in Berlin, said the more representative these talks are, the better. However, she added: "Clearly, people are only welcome if they can credibly show that they are prepared to reach their goals without violence." (rt/ez)