Hungary and Germany came to Serbia's rescue on Thursday as Belgrade's old ally Russia inched towards resolving a gas dispute with Ukraine that has caused a Europe-wide energy crisis.

Hungary accepted Serbia's request to sell two million cubic metres of natural gas daily to its southern neighbour, where supplies had run out amid freezing and snowy conditions, its Prime Minister Ferenc Gyurcsany announced.

The gas began streaming through cross-border pipelines at 4:00 pm (1500 GMT), Serbian news agency Beta reported.

Serbian President Boris Tadic later announced on state television that Belgrade had also forged a deal with the Germans to supply three million cubic metres of natural gas daily.

"From tomorrow (Friday), the gas supply will be stable," Tadic declared on RTS television.

The government in Belgrade had earlier appealed to Moscow to find an alternative route to provide emergency gas to Serbia, a close ally of the powerful, energy-rich former Soviet state.

Russia has refused to restart pumping until EU observers are in place to monitor Ukrainian gas flows.

The Hungarian and German deals mean Serbia's worst-affected regions, in particular the freezing northern province of Vojvodina -- most reliant on Russian gas piped through Ukraine and Hungary -- can breathe a sigh of relief.

Elsewhere in the Balkans, the region hardest hit by the crisis as it entered a third day, tens of thousands of homes were left without heating and several factories were forced to be closed in Bosnia and Bulgaria.

Authorities were to meet in Sarajevo amid fears Bosnia's alternative energy sources would dry up entirely within seven days.

Serbia has switched 90 percent of its heating plants to crude oil after Russian gas deliveries were completely halted at midnight on Tuesday.

However nine towns and cities, mostly in Vojvodina, are unable to make the changeover from gas, leaving the locations with an estimated 900,000 inhabitants without regular heating.

In Vojvodina, some factories stopped production after gas supplies finally ran out, making conditions too harsh for workers.

"We had to let our employees go because it's cold and it is impossible for them to work under these conditions," said Boris Milic of the Insert shoe factory, which employs 130 people in Sremska Mitrovica.

"We are trying to find an alternative means of heating," he told AFP on the telephone from the northwestern town.

Analysts said the crisis would prove a test of Serbs' patience with their "brothers" in Russia, which cut off natural gas deliveries to Ukraine on New Year's Day over a price dispute.

Unlike southeastern Europe, Ukraine and other parts of the continent had enough gas reserves for several months, journalist Dimitrije Boarov said in a newspaper column published Thursday.

"It turns out that by threatening Europe through this dispute with Ukraine, Russia is in fact punishing mostly Bulgaria and Serbia, which are its main traditional (allies) in the Balkans," Boarov wrote for the daily Danas.

"This gas shock that Serbia is going through is putting to the test its whole strategy of leaning on Russia," he said.

Serbia's leadership has already come in for heavy criticism from factions of the government over a December energy deal they consider too favourable for the Russian side.

That, they said, was made in exchange for Moscow's support for Belgrade in its opposition to Kosovo's independence from Serbia, declared by its ethnic Albanian majority 11 months ago.

But Tadic has sought to allay such fears about the latest crisis.

"Serbia is a collateral victim in all of this, like many other countries," the Serbian leader was quoted as saying in the daily Blic.

After an emergency cabinet session on Wednesday, Prime Minister Mirko Cvetkovic urged citizens to reduce electricity consumption, which many have turned to amid sub-zero temperatures and snow that carpeted the north.