US Secretary of Defence Donald Rumsfeld says there will not be a cleric-led government in Iraq similar to the one in Iran.
Rumsfeld: Iraqis need time to shape their future
The comment comes at a time when Washington is worried about Iraq's majority Shia population and its new-found religious freedom.
Just this week the US warned Iran not to try to destabilise the Shia community in Iraq, something it believes could interfere with Iraq's road to democracy.
But UK Foreign Secretary Jack Straw told BBC News Online that it was "a matter for the Iraqi people what kind of government they decide to elect".
In an interview with the Associated Press, Mr Rumsfeld said: "If you're suggesting, how would we feel about an Iranian-type government with a few clerics running everything in the country, the answer is: That isn't going to happen."
He said the Iraqi people needed time to determine for themselves how to organise a new government and elections.
Mr Rumsfeld's words contrasted with earlier comments by US President George Bush, who said it was up to the Iraqi people to decide who should rule them.
Muslims in government
Mr Bush said: "The form and leadership of that government is for the Iraqi people to choose.
"Anything they choose will be better than the misery and torture and murder they have known under Saddam Hussein."
US Secretary of State Colin Powell on Thursday also joined the debate over the make-up of Iraq's future government.
The US fears the rise of anti-American Islamic militancy
He said religious Muslims should not be precluded from governing Iraq.
"There are Islamic countries that are having elections -
Pakistan, Turkey. It's happening," Mr Powell said in an
interview with Dubai-based al-Arabiya television.
"Why can an Islamic form of government that has as its
basis the faith of Islam not be democratic?" he asked.
Shia Muslims are the majority religious group in Iraq, and they disagree on whether to embrace a secular government or an Iranian-style theocracy.
They recently protested against the US presence in Iraq in the central city of Karbala at the climax of a pilgrimage that attracted up to one million people.
Some US officials worry that the Islamic government in Iran, which is predominantly Shia, may seek to influence Iraq's post-war reshaping.
Iran has dismissed any suggestion that it is doing this.
A key backer of Britain's military involvement in Iraq, Jack Straw told BBC News Online that there was no reason to fear a Shia government.
"I wouldn't do anything about it personally - this is their choice," he said.
"What's so frightening about a state which is 'Islamic'?"
He said that democracy was now emerging in Iran after more than two decades of political turbulance.
"If you have democratic systems, which I want to see in Iraq, you have to accept the result of the ballot box," he said.