Pro-Russian forces in Slovyansk have shot down two Ukrainian helicopter gunships after Kiev launched an operation against the separatist stronghold that was condemned by Russia.

Ukraine's Defence Ministry said on Friday "unknown groups" had shot down two Mi-24s, while an Mi-8 transport helicopter was also damaged. The SBU security service earlier said one pilot was killed and another captured.

The SBU said man-portable missiles were used against one helicopter - proof that "trained, highly qualified foreign military specialists" were operating in the area "and not local civilians, as the Russian government says, armed only with guns taken from hunting stores".

Ukraine's military said later on Friday that they had arrested four separatists suspected of involvement in the downing of helicopter at a checkpoint outside Slovyansk shortly after a dawn attack.

Arsen Avakov, Ukrainian interior minister, said on his Facebook page that troops had taken control of nine checkpoints around Slovyansk despite heavy resistance. He blamed "terrorists" for the army's losses.

Vyacheslav Ponomarev, Slovyansk's separatist-appointed mayor, was quoted by Russia's Interfax agency as saying one pilot was taken hostage.

Russian TV, available in Donetsk and parts of eastern Ukraine, broadcast footage of the captured pilot, who was injured.

Russia has warned Ukraine it must not launch attacks on Ukrainian civilians, who it says have legitimate concerns. Kiev says the separatists are being boosted by Russian soldiers.

Slovyansk has become a separatist stronghold, and there have been regular clashes between pro-Russians and Ukrainian troops. Last week, Ukraine sent troops to dismantle several roadblocks but withdrew forces soon after.

Slovyansk's separatists have held seven European inspectors from the Organisation for Security and Cooperation in Europe for a week. They had been sent as part of a deal world powers struck in Geneva to ease tensions in Ukraine.

Dmitry Peskov, a spokesman for Russian President Vladimir Putin, on Friday said the "punitive operation" in Slovyansk had destroyed any hope of keeping the deal alive.

Russia had sent an envoy to southeast Ukraine to negotiate the realease of the monitors, Peskov said.