EU member states and Turkey have signed a transit agreement for the EU and US-backed Nabucco gas pipeline, which aims to reduce Europe's energy dependence on Russia.
Transit countries Turkey, Bulgaria, Romania, Hungary and Austria signed the accord in the Turkish capital Ankara.
The €7.9bn ($10.99bn) project, which has been designed to supply Europe with gas from the Caspian and Middle East, has suffered years of delays.
"I believe this pipeline is inevitable, not impossible," European Commission president Jose Manuel Barroso said.
The project had suffered delays due to objections from the US to Turkey's wishes to enable Iranian gas to flow through the pipeline.
US special energy envoy Richard Morningstar has renewed US opposition to possible use of Iranian gas in Nabucco until Washington can agree the terms of Iran's nuclear programme.
Even though the deal has been signed, questions over supply and financing might cause problems for the future of the Nabucco pipeline project.