WRAPUP 2-EU, Turkey sign Nabucco gas transit agreement

* Nabucco "inevitable not impossible" - Barroso

* OMV sees Azeri, Iraqi gas carried

* Turkey says Iran should be included in Nabucco

* Turk PM says Russia gas can be transported too

(Adds analyst comment para 13-15)

By Thomas Grove and Orhan Coskun

ANKARA, July 13 (Reuters) - EU countries and Turkey on Monday signed a transit deal for the Nabucco gas pipeline, aimed at cutting Europe's energy dependence on Russia, a major political step backers say will boost the project.

Transit countries Turkey, Bulgaria, Romania, Hungary and Austria signed the accord for the 7.9 billion euro ($10.99 billion) European Union and United States-backed project, which aims to supply Europe with gas from the Caspian and Middle East.

The agreement irons out details over transit and tax issues for hosts of the pipeline.

"The Nabucco project is being labelled a pipe dream," Turkish Prime Minister Tayyip Erdogan said at the ceremony. "This project will be a success story that will prove the doubters wrong."

The signing of the long-delayed transit agreement is a major boost for a project that has been marred by political infighting and is still dogged by questions over supply and financing.

Progress of a rival Russian plan has also added doubts to the feasibility of Nabucco, which has been unable to find sufficient gas to put through the pipeline.

But on Monday, backers sought to dispel such doubts.

"We are starting to confound the sceptics, negotiations once seemed irrevocably blocked, but now we have an agreement and I believe this pipeline is inevitable, not impossible," European Commission President Jose Manuel Barroso said.

"Politically it is all wrapped up. There is nothing more to be done because now we are at a totally commercial stage that hinges on the open season ... and the construction of the pipeline," EU Energy Commissioner Andris Piebalgs said.