KIEV: Pro-Russian rebels downed a Ukrainian helicopter Thursday, killing 12 soldiers including a general and undermining president-elect Petro Poroshenko's fervent vow to crush the bloody seven-week insurgency roiling the industrial east.

The militants' success was followed by the surprise admission by one of their leaders that 33 of more than 40 rebels killed during a raid on a Donetsk airport at the start of the week were Russian nationals from Muslim regions such as Chechnya.

The revelation challenged President Vladimir Putin's rejection of Russian links to the separatist drive and supports Kiev's claims that the rebels do not represent the true will of the miners and steel workers who have turned the east into the economic engine of Ukraine.

Separatists had earlier Thursday also confirmed they were holding four unarmed European monitors in the same region from which the Mi-8 helicopter gunship was shot out of the sky with a sophisticated surface-to-air missile.

Western-backed Poroshenko – winner of 54.7 per cent of Sunday's vote and due to be inaugurated as president June 7 – must further avert another showdown with Russia that could see his economically teetering country cut off from gas supplies by the start of next week.

Yet all attention Thursday was fixed on Slavyansk – an industrial city of 120,000 mostly ethnic Russians that was the first of a dozen towns and cities seized by the rebels in response to the February ouster in Kiev of a pro-Kremlin president.

“I just received information that near Slavyansk, the terrorists – using a Russian man-portable air defence system – shot down our helicopter,” acting President Oleksandr Turchynov told parliament.

Those killed included General Volodymyr Kulchytskiy and six members of the National Guard force made up of volunteers and interior ministry troops.

A separatist spokesman had earlier told Russian news agencies that the helicopter was downed in a fierce battle that was still raging on the southern outskirts of Slavyansk.

The unnamed spokesman said that “several houses” belonging to civilians were on fire as a result of what he said was military activity.

Thursday's death toll is the highest since Ukraine lost 18 soldiers during hours of heavy fighting in the same Donetsk region on May 22.

Poroshenko – a 48-year-old confectionery tycoon who once enjoyed good relations with top Russian officials – reached out to Vladimir Putin on Wednesday by announcing that he intended to speak to the Kremlin chief when they both attend D-Day commemorations in Normandy on June 6.

The talks would be the first between the leaders of the two countries since a popular uprising chased a Kremlin-backed regime from power in February and installed a new administration intent on breaking Russia's historic hold on Ukraine.

'National disaster'

Russia kept up its diplomatic offensive by urging Western powers “to use all their influence on Kiev to stop Ukraine from descending into a national disaster.”

“The international community awaits from Kiev an immediate ceasing of military activities in the east of the country and the withdrawal of troops,” the Russian foreign ministry said.

But it was the militants' actions Thursday that were of greatest concern to the West.

The self-proclaimed “people's mayor” of Slavyansk announced that the four civilian monitors from the Organisation for Security and Cooperation in Europe (OSCE) had been detained on suspicions that they were spies.

“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,” Vyacheslav Ponomaryov told Russia's Interfax news agency.

He later told Russia's RIA Novosti agency that they were being held in a town on the eastern outskirts of Donetsk called Makiivka.

But Ponomaryov avoided called them “prisoners of war” – the tag militants assign to seven OSCE monitors held in Slavyansk for eight days and released on May 3. An eight member of that group was set free due to ill health after two days.

A source at the organisation told AFP that the missing team – a Dane, an Estonian, a Turk and a Swiss national – included one woman and that negotiations for their release had been ongoing for some time.

Gas deadline

Cash-strapped Ukraine appeared to avert the immediate threat of a Russian gas cut-off when the European Union announced that a round of last-gasp talks had been urgently set up for Friday in Berlin.

Kiev had until midnight Thursday to pay Russia $2 billion under an EU-brokered agreement or face a halt in gas supplies next week that would also hit parts of Europe.

Russia and Ukraine launched their third gas war in less than a decade after Moscow decided to cancel its previous rebates and nearly double the price it charges Kiev for gas after the Kremlin-backed president's fall.