Nabucco is unlikely to provide more than 5% of EU needs
A deal to supply the EU with 10bn cubic metres of Turkmen gas per year from 2009 has been hailed by officials as "an important step".
The agreement will boost the EU-backed Nabucco pipeline - planned to reduce reliance on Russian gas, which accounts for a quarter of EU supplies.
The Turkmen gas will only make up a small percentage of EU demands and it is not clear how it will reach Europe.
Nabucco is due to be built in 2010 and the first gas will flow in 2013.
A spokeswoman for EU External Relations Commissioner Benita Ferrero-Waldner said she had received a commitment during a meeting in Central Asia last week with Turkmen President Kurbanguly Berdymukhamedov.
The annual guarantee to the EU will come as a relief to Nabucco's backers, as the Turkmen leader had already agreed a major deal with Russia.
"For the first time, we have a concrete volume for Nabucco which comes from Turkmenistan alone", the spokeswoman said.
What is less clear is how the gas will reach Europe.
The spokeswoman said it was now up to companies to become involved, possibly by building a mini-pipeline to connect Azerbaijan-owned installations in the Caspian Sea with Turkmen rigs.
When complete, Nabucco will extend 3,300 km (2,050 miles) from an Austrian hub at Baumgarten to Turkey's eastern borders with Iran and Georgia.
It is expected to carry a maximum of 31bn cubic metres a year, no more than 5% of EU needs.