Pro-Russian separatist have shot down a Ukrainian army helicopter, killing 14 soldiers including a general.

Government forces continue to press ahead with an offensive to crush rebellions in the east swiftly following the election of a new president.

Acting president Oleksander Turchynov said the helicopter, which had been carrying supplies in eastern Ukraine, had been brought down by anti-aircraft fire from near the town of Slaviansk.

It was one of the heaviest losses inflicted by the separatists on the army in two months of unrest in Ukraine's eastern regions.

Meanwhile, a rebel leader in Donetsk acknowledged that some of his fighters who died in the government offensive there had been "volunteers" from Russia.

He said their bodies were being returned home across the border.

His claim followed weeks of accusations from Kiev of Russian involvement in the uprising.

Around 50 rebels were killed earlier this week in a fierce assault by government forces in Donetsk.

Interior minister Arsen Avakov accused the government of Russian President Vladimir Putin of being behind the airport violence.

Weapons collected at the airport after the rebels were forced out by airstrikes and a paratroop assault had been brought in from Russia, he said.

"These are not our weapons - they were brought from Russia. Serial numbers, year of production, specific models ... I am publishing this photograph as proof of the aggression of the Putin regime," Mr Avakov wrote on his Facebook page.

The Ukrainian government has long asserted that Russia has fomented separatists rebellions in the east with a view to bringing about dismemberment of the country.

Russia denies this but they also allege that it is failing to stop Russian fighters from crossing the long land border into Ukraine together with truckloads of guns and live ammunition.

Defence Minister Mikhailo Koval said: "We have put all our forces and equipment into the anti-terrorist operation. We have covered the whole state border."

Russian Foreign Minister Sergei Lavrov yesterday accused the West of pushing Ukraine into "the abyss of fratricidal war", and reiterated his call for an end to Kiev's military offensive.

Four OSCE observers held in Slaviansk

The leader of pro-Russia rebels in Slaviansk has confirmed that his forces have detained four OSCE observers since Monday.

"The group of four people who went missing south of Donetsk - we know where they are, they are all fine.

"We had told them not to travel anywhere for a time but these four turned out to be very keen. Of course they were detained," said Vyacheslav Ponomaryov, the self-proclaimed "people's mayor" of Slaviansk, in comments to the Interfax news agency.

"No one arrested them. We detained them. Now we will work out who they are, where they were going and why, and we will let them go," Mr Ponomaryov said.

He suggested that the team could have been involved in espionage.

The observers from the Vienna-based Organisation for Security and Co-operation in Europe, a Dane, an Estonian, a Turk and a Swiss national, went missing at a rebel roadblock outside Donetsk on Monday.

The Ukrainian authorities had already said yesterday that the four observers had been seized by pro-Russian rebels.

A second group of 11 observers was detained in the same region of Donetsk yesterday but the OSCE later said it had managed to re-establish contact with them.

Swiss President and OSCE chief Didier Burkhalter criticised the detention of observers as "acts of sabotage".

In a statement, Mr Burkhalter called for their "immediate and unconditional release".

Last month seven OSCE observers were detained by Pro-Russian activists in Slaviansk for eight days, before being released.