Brussels - "A strange game: the only way to win is not to play."

The quote comes from 1983 movie "War Games," in which a computer-obsessed teenager averts World War Three by teaching a US missile-control computer the impossibility of winning a nuclear conflict.

And it could well apply to the current struggle over gas supplies between Russia and Ukraine, as the collapse of deliveries to Europe threatens to wreck both sides' credibility across the continent.

"Russia and Ukraine are discrediting themselves as reliable energy sources," one EU diplomat told Deutsche Presse-Agentur dpa after an emergency meeting of EU representatives in Brussels on Monday.

Europe, Russia and Ukraine all rely heavily on one another in the lucrative trade in natural gas.

Russia is the EU's single biggest source of the fuel. Roughly one quarter of all the gas burnt in the EU comes from Russian sources - and 80 per cent of that gas passes through pipelines in Ukraine.

In return, natural gas is one of Russia's biggest export earners, with oil and gas together accounting for two-thirds of its income.

And according to the Oxford Institute for Energy Studies, Ukraine makes 3 billion dollars a year by levying a transit fee on westbound gas - giving it a keen interest in keeping the fuel flowing.

Not surprisingly, both Moscow and Kiev have spared no efforts to convince the EU that the other side is in the wrong in their dispute, which sharpened dramatically on Tuesday morning with gas shut-offs hitting countries across Central and Eastern Europe.

The diplomatic push has reached the highest levels, with Russian Prime Minister Vladimir Putin and Ukrainian premier Yulia Tymoshenko both calling the head of the European Commission as the row began, to promise that the dispute would not lead to gas shortages in Europe.

But those pledges collapsed on Tuesday as gas supplies nosedived in a number of EU states - to the accompaniment of a hail of accusations between Moscow and Kiev.

The recriminations cut no ice with the EU, which flatly refused to get involved in the question of who was to blame, instead condemning the cuts as "completely unacceptable" and demanding that both sides immediately return to negotiations.

"Such a drastic reduction in supply is not a standard way of resolving commercial disputes," Czech deputy premier Alexandr Vondra said Tuesday, likening the dispute to "holding countries hostage."

Nor did they impress European commentators, who variously interpret the row as a Kremlin-inspired blow at the pro-Western government of Ukraine's President Viktor Yushchenko or a settling of scores between political rivals Yushchenko and Tymoshenko.

Diplomatically, the crisis comes at a time when neither Russia nor Ukraine can well afford to lose more credibility in the EU's eyes.

Ukraine has already seen its bid to join the bloc stall because of the paralysing effects of its long-running political instability.

Russia, meanwhile, is still suffering from the diplomatic fall-out of its August invasion of Georgia and its subsequent decision to recognize the independence of Georgia's separatist provinces.

And the fact that the current crisis is a virtual replay of a dispute in 2006 has sparked a flurry of calls for the EU to find a way of getting gas without the help of either neighbour - a result which would add a major commercial cost to the diplomatic one.

The Czech government, which currently holds the EU's rotating presidency, is now making the diversification of energy supplies one of the EU's top priorities over the next six months, with pipelines cutting out both Russia and Ukraine heading the list, Vondra said.

And the dispute gives a further boost to EU plans to increase its use of renewable fuels such as wind, wave and plant-based energies, all sources which can be produced at home, EU officials point out.

With the EU debate shifting ever further in favour of those who say that the way to solve the problem is to stop relying on either Russia or Ukraine, the only winners in the Moscow-Kiev power game look set to be those energy sources - countries such as Algeria and Azerbaijan, and technologies such as wind power - who are not playing.