"This is a complete breakthrough," said a senior EU official involved in the tough negotiations with Turkey. "The Turks have accepted our terms. There is no conditionality."

The Nabucco project is the focus of an energy-supply debate that ranges Russia against the EU, and involves Turkey, Germany, Austria, Azerbaijan, and the regimes of central Asia in the effort to secure Europe's gas needs while curbing the hold Moscow and the gas monopoly Gazprom is seen as having over the current supply situation. The 3300-km long Nabucco route from eastern Turkey's to Europe's main gas hub outside Vienna, would be a main route for transporting gas to Europe outside the control of Gazprom.

The plan to build Nabucco reached a deadlock between the EU and Turkey over the pipeline transit agreement: as more than half the pipeline will be in Turkey, this makes the country an effective 'gatekeeper' of Europe's energy supplies. The Ankara government has been driving a hard bargain, requiring 15% of the transit gas at discounted rates as well as a transit tax. This, say EU officials and the six-company consortium that is to build and run the pipeline, would render Nabucco financially unviable.

The stalemate was broken at a summit in Prague on 8 May between the EU and the countries involved. "The 15% demand has gone," Andris Piebalgs, the EU commissioner for energy, told the UK's Guardian newspaper. "We've agreed on cost-based transit. We're very close to a conclusion." A senior Czech official at the summit likened the negotiations to "bargaining in an Istanbul souk," while an EU envoy to the region worried that "nothing is done until it's done".

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However, the European Commission president José Manuel Barroso said President Abdullah Gül of Turkey assured him the deal would be signed within weeks, although the Turkish leader indirectly linked any Nabucco deal with progress on Turkey's negotiations on joining the EU. The negotiations are being blocked by Greek Cypriots, while several big EU states are quietly happy to see Turkey's EU bid frozen. But Barroso and others insisted that Ankara was not setting conditions for a Nabucco agreement.

The EU imports about one-third of its gas needs, or 140bn cum/yr, from Russia. The "southern corridor", formed by Nabucco and two other pipelines, could transport 60bn cum/yr, or 10% of estimated requirements by 2020.

It is understood that the EU is also thinking about supporting a pipeline under the Caspian Sea connecting Turkmenistan and central Asia to Azerbaijan. If Nabucco is to happen, it will initially need the gas from Azerbaijan's BP-run Shah Deniz-2 field, although officials in Brussels view Turkmenistan, with its vast gas deposits, as the key to Nabucco's longer-term viability. Russia is understood to be urging the central Asians and Azerbaijan hard to try to put a stop to Nabucco and retain control of all the supply routes to the west. Turkmens attended the Prague summit, but declined to commit, apparently deciding to try to play the Russians off against the Europeans.