EU nations and Turkey said Friday they will sign a key intergovernmental agreement in Ankara on July 13 on Europe's flagship Nabucco gas pipeline project, but key issues still need to be resolved.

Turkish officials said that invitations had been sent to the relevant parties and one official in Ankara told AFP on condition of anonymity that Turkish Prime Minister Recep Tayyip Erdogan would host the signing ceremony.

Foreign Minister Ahmet Davutoglu said on a visit to Romania that Nabucco, which is planned to reduce Europe's dependency on Russian gas imports, was a "priority" and a "very important, strategic project" for his country.

He added that it would "ensure cooperation between the participating states and will solve the general energy problem that faces mankind."

The European Commission, which is not a signatory to the agreement but has been a key proponent of the pipeline and could sign up as an observer once legal details are ironed out it, confirmed that it had been invited.

"The commission has received an invitation to the signing ceremony of the intergovernmental agreement on the Nabucco pipeline on July 13 in Ankara," a spokesman on energy issues told reporters.

He declined to provide details about the agreement, saying only that it would provide a legal framework including deciding on the allocation of gas for each country that the pipeline would go through.

The agreement is due to be signed by Austria, Bulgaria, Hungary, Romania and Turkey, all the countries through which Nabucco will flow.

The 3,300-kilometre (2,000-mile) pipeline is to pump gas from the Caspian Sea to Austria while bypassing Russia -- the main source of Europe's gas.

The project is planned to become operational in 2014 and estimated to cost 7.9 billion euros (10.6 billion dollars).

It is in direct competition with Russia's South Stream project, developed by Russian gas giant Gazprom and Italy's Eni, which will channel Russian gas through Bulgaria to Western Europe under the Black Sea.

Critics have questioned the availability of gas sources for Nabucco, and the project was cast into further doubt in May when key gas suppliers Turkmenistan, Kazakhstan and Uzbekistan held off their support at a meeting in Prague.

Russia also raised concern among Nabucco proponents when it clinched a key deal to buy gas from Azerbaijan, another gas-rich Caspian Sea state.

The signing of the agreement between Nabucco's shareholders has been delayed by Turkey's demands to use a percentage of Nabucco's capacity of 31 billion cubic metres of gas for domestic use or even for re-export.

There was no clear confirmation on Friday from either side that the issue had been resolved.

Turkish officials visiting Brussels last week also hinted they might back off from the deal if EU accession talks are further delayed.

Turkey has so far opened only 11 out of 35 negotiation chapters with the EU. The chapter on energy significantly remains blocked.

A diplomatic source close to the Swedish EU Presidency told AFP in Brussels that opening this chapter by end-2009 will be a difficult task due to rising anti-Turkish feelings within the bloc.

"We talk about Nabucco but at the same time the chapter on energy is not discussed. Why not talk about it?" Turkey's Erdogan asked during a visit to Brussels last month.

"This is the reason why we have difficulties (with Nabucco). Our European friends have a unilateral approach to this issue," seeking Ankara's support but giving nothing in return, he added.