Brussels  - The European Union on Friday urged Russia immediately to restore gas supplies to Europe after satisfying one of Moscow's key demands by sending a team of observers to Ukraine.

But the taps remained shut by late afternoon, as skirmishes erupted between representatives of the two countries' state-owned monopolies, Gazprom and Naftogaz Ukraine.

"The team (of observers) has begun its work," the EU's executive arm, the European Commission, said in a statement.

"This means that all the conditions agreed between the leaders of the European Union, Russia and Ukraine are in place for the immediate restart of gas supplies from Russia that are destined for European customers," the commission said.

Officials in Brussels said that as soon as the taps were re-opened, it would take about three days for the gas to reach European customers.

But a deal brokered by the EU on Thursday and Friday still hung in the balance as Gazprom and Naftogaz failed to reach a written agreement on the deployment of additional Russian and Ukrainian observers to each other's territory.

"Naftogaz is still playing for time and shows no interest in a prompt resolution of the crisis," a Russian government source was quoted as saying in Moscow.

The EU mission, consisting of about 20 senior EU officials and experts from the European gas industry, is tasked with verifying that no Russian gas is siphoned off by Ukraine while on its way to European customers.

Russian allegations that Ukraine was stealing its gas amid a price war between the two sides had prompted Gazprom to interrupt all supplies to Europe on Wednesday, forcing many countries to tap into their reserves.

The EU observers are to act as independent monitors at the six main natural-gas pipeline crossing points from Russia to Ukraine, and at the Naftogaz headquarters.

Earlier Friday, EU officials said that Russian Prime Minister Vladimir Putin had vowed to restart supplies as soon as the mission, agreed in telephone conversations overnight, would get under way.

Czech Prime Minister Mirek Topolanek, whose country holds the rotating presidency of the EU, was expected to try and broker the final details of the monitoring mission during a visit to Kiev.

Topolanek's mission is intended to "discuss specific details of the functioning of the monitoring group and to make sure the observers are deployed immediately at points that are relevant for the transit of Russian gas via Ukraine to the EU, so that gas supplies can be restored as soon as possible," an EU presidency statement said.

Russia, meanwhile, continued its diplomatic push with Putin calling Chancellor Angela Merkel of Germany, Gazprom's biggest client.

EU member states in Central and Southern Europe have already been hard hit by Russia's shut-off of all gas supplies through Ukraine, with Slovakia relying on gas supplies from Poland and EU neighbour Serbia seeking emergency help from Germany, Austria and Hungary.

Gas experts from EU member states met in Brussels on Friday to discuss emergency support for those countries hardest hit by the crisis, while EU energy ministers are set to hold yet another emergency meeting on the issue on Monday.

About a quarter of the EU's gas needs are met by Russian gas, 80 per cent of which reaches European clients via Ukraine.

Russia and Ukraine also failed Friday to make any headway in the long-standing contractual dispute that has sparked the gas cuts. (dpa)