"We want an agreement and we will spare no effort to achieve one by the August deadline," Mr Barzani told the Guardian. "But it will be impossible for us to accept a constitution if we know that the demands and the aspirations of the Kurdish people on federalism, Kirkuk and the control of natural resources in our region have not been met."

He said Kurds believed that foreign policy, defence and monetary policy should be handled by the federal government in Baghdad, but "everything else should go to the regions".

The Kurds' wish to decentralise is opposed by the Sunni Arabs, who want to see a strong central government. "Our patience with the Sunnis has a limit," said Mr Barzani, who heads the newly unified Kurdish administration.

He said Kurds could never accept a Sunni Arab demand to describe Iraq as being part of the Arab nation. "If they want to say that the Arab part of Iraq is part of the Arab nation, that is fine. Then we can say the Kurdish part of Iraq is part of the Kurdish nation. But somehow I don't think they'll agree to that."

Iraq's interim law provides for a six-month extension for a constitution, but US officials say there must be no slippage. The eagerness to see a deal done on time has led to accusations of heavy-handedness and raised concerns about the potentially negative effects of a "rushed document".

"The US and the UK are working behind the scenes, dealing with all the groups, saying it should be like this and it should be like that," said Mr Barzani. "Like the Sunnis they seem to want to centralise power in Baghdad - it's very disappointing."

Mahmoud Othman, a member of the constitutional committee in Baghdad, said US and UK officials were being governed by their domestic agendas. "They and the British are meeting individually behind the scenes with members of the committee. It's not right and is counterproductive. If they have something to say, why don't they come and address the whole committee?"