KIEV, April 24 - Ukrainian forces killed up to five pro-Moscow rebels on Thursday as they closed in on the separatists' military stronghold in the east, and Russia launched army drills near the border in response, raising fears its troops would invade, Reuters reported.

The Ukrainian offensive amounts to the first time Kiev's troops have used lethal force to recapture territory from the fighters, who have seized swathes of eastern Ukraine since April 6 and proclaimed an independent "People's Republic of Donetsk.”

Ukraine's acting president accused Moscow of supporting "terrorism at the state level" against his country for backing the rebels, who the government blames for kidnapping and torturing a politician found dead on Saturday.

The Ukrainian Interior Ministry said its forces backed by the army had removed three checkpoints manned by armed groups in the separatist-controlled city of Slavyansk.

"During the armed clash up to five terrorists were eliminated," it said in a statement, adding that one person had been wounded on the side of the government forces.

A rebel spokeswoman in Slavyansk said two fighters had died in a clash in the same area, northeast of the city center. Slavyansk’s separatist self-proclaimed mayor, Vyacheslav Ponomaryov, quoted on a local news site, said one man was shot dead and another badly wounded on the northeastern outskirts of the city. He said the dead man had been unarmed.

The Kremlin, which says it has the right to invade its neighbor to protect Russian speakers, has built up forces on Ukraine's border - estimated by NATO at up to 40,000 troops.

Defense Minister Sergei Shoigu announced Moscow had launched military drills near the border in response to "Ukraine's military machine" and NATO exercises in eastern Europe. Kiev demanded an explanation within 48 hours of action on the border.

Russia already seized and annexed the Crimean peninsula from Ukraine last month after President Vladimir Putin overturned decades of post-Cold War diplomacy by announcing the right to use military force in neighboring countries.

An invasion of mainland Ukraine's industrial heartland would be a far more serious action. It had seemed beyond contemplation only weeks ago but now looks like a real threat, although the full extent of Putin's territorial ambitions remains a mystery.

In St Petersburg, Putin said that if the authorities in Kiev had used the army in eastern Ukraine, it would be a very serious crime against their own people.

"It is just a punitive operation and it will of course incur consequences for the people making these decisions, including (an effect) on our interstate relations," Putin said in a televised meeting with regional media.

Washington criticized the Russian drills on the frontier. It was "exactly the opposite of what we have been calling on the Russians to do, which is to de-escalate the situation," Pentagon spokesman Colonel Steve Warren said. (rt/ez)