KIEV (AFP) - Ukraine's vow to crush the bloody insurgency in the industrial east of the country suffered a major blow Thursday when pro-Russian rebels downed an army helicopter, killing 12 soldiers including a general.
The militants shot the Mi-8 helicopter gunship out of the sky with a sophisticated surface-to-air missile, which the White House said raised concerns about the rebels being supplied "from the outside".
One of the separatists' leaders made a surprise admission on Thursday that 33 out of more than 40 rebels killed during a raid on a Donetsk airport this 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.
Moscow meanwhile called on Kiev to impose an immediate ceasefire and urged the West to use its influence to prevent "a national disaster" in Ukraine.
"The international community awaits from Kiev an immediate ceasing of military activities in the east of the country and the withdrawal of troops. Without that, achieving peace in Ukraine is impossible," the foreign ministry said in a statement.
The country's Western-backed president-elect Petro Poroshenko - winner of 54.7 percent of Sunday's vote and due to be inaugurated on June 7 - needs to avert another showdown with Russia that could see his economically teetering nation cut off from gas supplies by the start of next week.
But cash-strapped Ukraine appeared to avert the immediate threat of a gas cut-off when the European Union announced that a new round of talks had been urgently set up for Friday in Berlin.
Rebels use Russian-made system
Yet all the Kiev government's attention on 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.
It was the highest death toll since Ukraine lost 18 soldiers during hours of heavy fighting in the same Donetsk region on May 22.
White House spokesman Jay Carney said while still trying to verify the reports, "we are concerned that this indicates separatists continue to have access to advanced weaponry and other assistance from the outside." 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.
Mr Poroshenko - a 48-year-old confectionery tycoon who once enjoyed good relations with top Russian officials - reached out to Mr 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 installed a new administration intent on breaking Russia's historic hold on Ukraine.
Suspected as spies
In Slavyansk the self-proclaimed "people's mayor" announced that 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 the pro-Russian "prime minister" of the self-declared Republic of Donetsk added confusion to the fate of the OSCE monitoring team, raising uncertainty about their whereabouts.
"We don't have any information on this subject. We do not know where they are and we are looking for them," said Alexander Borodai, adding that the Slavyansk mayor had a tendency "to exaggerate things".
A source at the OSCE 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 have been going on for some time.
With talks set for Friday Kiev has been relieved from a midnight deadline to pay Russia US$2 billion (S$2.5 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.
Ukraine refused to pay in protest and has since baulked at the terms of an interim deal negotiated with the help of a top EU energy official.
About 15 per cent of all gas consumed in Europe is pumped in from Russia through Ukraine and analysts said it was in both sides' interest to find a compromise.