The White House said on Thursday that it had concerns that separatist rebels were being supplied with advanced weapons, after the shooting down of a Ukrainian military helicopter.

While the US administration did not directly accuse the Kremlin of providing arms to pro-Russian militants, it called on Russia to do all it could to rein them in.

"We are disturbed by the ongoing violence in eastern Ukraine," said White House spokesman Jay Carney. "We are concerned that this indicates separatists continue to have access to advanced weaponry and other assistance from the outside."

The Mil-8 transport helicopter had been returning from a troop rotation when it was shot down on the outskirts of Slovyansk, reportedly by rebels using a shoulder launched missile.

At least 12 troops were reported dead, the highest loss of life for government troops since 18 were killed in heavy fighting on May 22 around Donetsk.

Among those killed in the downing of the helicopter was National Guard training chief General Serhiy Kulchytskiy. While the National Guard later claimed that the attackers who shot at the aircraft had been killed, this was denied by the self-declared Mayor of Slovyansk, Vyacheslav Ponomaryov.

Earlier, on Thursday Ukraine's election commission confirmed that billionaire Petro Poroshenko - the so-called "Chocolate King" - is the winner of Sunday's presidential vote with 54.7 percent.

Gas summit

Meanwhile, preparations were underway for a summit in Berlin on Friday afternoon, aimed at averting the threat of a gas cut-off to Ukraine.

Talks have been brokered by the European Commission following Moscow's threat to stop supplying gas unless Ukraine makes a pre-payment for gas supplies by June 2.

Russia's state controlled gas firm Gazprom has said Ukraine's debt for gas supplies will have risen to about $5.2 billion (3.8 billion euros) by June 7, unless Kyiv begins to pay it off. A disruption of supplies to Ukraine - which is a transit state for about half of the gas that Russia sells to the EU - would possibly lead to a cut-off for third party nations that include EU members states.

rc/jm (AFP, AP, dpa, Reuters)