Iraq has offered to supply enough gas to fill half the capacity of the proposed Nabucco pipeline, the prime minister said, giving the project a boost even as heads of government met to sign a historic agreement approving the plan.

The offer from the Iraqi government to supply 15bn cubic metres a year of gas by 2015 helps address the greatest obstacle to the 3,300km pipeline from eastern Turkey to Austria: the prospect of there not being enough gas to fill it.