UPDATE 1-Turkish PM says wants Nabucco to transport Iran gas

* Turkey says Iran should be included in Nabucco

* Qatar and Russia can take role in Nabucco

(Adds quotes, background)

ANKARA, July 13 (Reuters) - Turkey's Prime Minister Tayyip Erdogan said on Monday Turkey wants Iranian gas to be transported to Europe via the planned Nabucco pipeline "when conditions allow", despite U.S. opposition.

European Union governments and Turkey will sign transit agreements in Ankara on Monday for the EU and U.S.-backed Nabucco pipeline, which aims to reduce Europe's energy dependence on Russia by transporting gas from the Caspian and Middle East through the planned pipeline from 2014.

"We desire Iranian gas to be included in Nabucco when conditions allow," Erdogan told a gathering of Nabucco partner countries and regional countries including Iraq and Georgia.

United States special energy envoy Richard Morningstar on Sunday reiterated Washington's opposition to the possible use of Iranian gas in the Nabucco pipeline.

Transit countries Turkey, Bulgaria, Romania, Hungary and Austria will sign the accord later on Monday, giving the 7.9 billion euro project a major political boost. But questions over supply and financing still plague Nabucco's feasibility.

Erdogan also said Qatar could take a major role in the project through a liquefied natural gas terminal in Turkey. He also believes Russian gas can be transported to the European markets via Nabucco.

The European Union has supported the project as a way of reducing its reliance on Russian gas, with possible suppliers to include Iraq, Egypt, Iran, Azerbaijan, Russia and Turkmenistan.

The Nabucco project has been unable to find sufficient throughput for the 31 billion cubic metre pipeline, which is competing with the rival Russian-backed South Stream project to feed growing European consumption.

Predominantly Muslim Turkey, which aspires to join the European Union, hopes Nabucco will strengthen its position as an energy hub for the West and advance its EU bid. (Reporting by Thomas Grove and Orhan Coskun; Editing by Ibon Villelabeitia and Jon Boyle)