Alan Waddams told the Azerbaijani ANS TV channel that EU representatives, currently on a visit to Turkmenistan, said Ashgabat had agreed to export gas to Europe via Nabucco, but if the Turkmen side changed its mind, then Iran could step in as an alternative.
The $7-8 billion Nabucco pipeline, backed by the EU and the U.S., is expected to link energy-rich Central Asia to Europe through Turkey, Bulgaria, Romania, Hungary and Austria.
The European diplomat said Turkmen President Gurbanguly Berdymukhammedov announced at the meeting his agreement for gas exports.
Some experts say that without the support of Turkmenistan, a major natural gas producer in Central Asia, the Nabucco project is unrealistic.
Construction on the pipeline with a projected annual capacity of between 20 and 30 billion cubic meters has been tentatively scheduled to begin in 2010.
Azerbaijan, Iran, Turkmenistan and, more recently, Iraq have been seen as possible suppliers for the project. Iraq is being backed by the United States.
Russia is skeptical about the successful implementation of the Nabucco pipeline in the near future citing insufficient gas supplies.
In what was widely seen as a major blow to the Nabucco project, Russia, Turkmenistan and Kazakhstan signed a deal in December to supply the Asian states' Caspian gas via Russia. Moscow also reached deals with Bulgaria and Serbia earlier this year on the South Stream pipeline to pump Central Asian gas to Europe.