Several European countries and Turkey launched the Nabucco project in an attempt to reduce their reliance on Russian natural gas, but the partners recently signalled that Russia's Gazprom could also supply gas to the pipeline.

The US had initially reacted coolly to the possibility of Gazprom's involvement in Nabucco.

"If our European partners and the companies feel secure... and they feel they can structure a contract that is commercially viable that still allows them to diversify their gas supplies... that is fine," US Deputy Assistant Secretary for European and Eurasian Affairs Matthew Bryza told Reuters in an interview.

"We want them (the project participants) to have a good relation with Russia."

The 3300 kilometre, €4.6 billion ($6.2 billion) Nabucco project, led by Austria, aims to carry Caspian and Middle Eastern gas to Europe via Turkey and the Balkans.

Bryza reiterated Washington's firm opposition to Iranian involvement in the project, which aims to pump Turkmen, Azeri and eventually Kazakh natural gas to European markets.

The US, which has no diplomatic ties with Iran, is leading Western efforts to force Tehran to abandon its atomic programme. Washington accuses Tehran of trying to build nuclear weapons. Iran says its programme has purely peaceful aims.

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US 'not opposed' to Russian Nabucco role