The US ambassador to Iraq has told Shia leaders that the US government does not want Ibrahim Jaafari to remain prime minister, senior Shia politicians say.
Ibrahim Jaafari's government has been criticised for poor performance
Zalmay Khalilzad said President George W Bush "doesn't want, doesn't support, doesn't accept" the retention of Mr Jaafari, Rida Jawad al-Takki said.
Mr Jaafari's spokesman accused the US of trying to subvert Iraqi sovereignty.
The Shia United Iraqi Alliance chose Mr Jaafari as its candidate in February after winning December's election.
But Kurdish and Sunni Arab parties have rejected the UIA's nomination and have threatened to boycott a national unity government unless it is withdrawn.
The delay in forming a government is also thought to be partly responsible for fuelling the increasing sectarian violence which has struck Iraq since last month's bombing of the al-Askari shrine at Samarra, one of Shia Islam's holiest.
Mr Takki said the US ambassador had passed on his government's dissatisfaction with Mr Jaafari at a meeting with the leader of the Supreme Council for the Islamic Revolution in Iraq, Abd al-Aziz al-Hakim, on Saturday.
"George Bush sent a message via Khalilzad to Abd al-Aziz al-Hakim, as head of the Alliance, telling him that George Bush doesn't want, doesn't support, doesn't accept Ibrahim Jaafari as prime minister," he said.
The US embassy in Baghdad confirmed Mr Khalilzad had met Mr Hakim on Saturday, but denied he had made such a demand.
"Iraq's democratically-elected Council of Representatives has the mandate of affirming a prime ministerial nomination," an embassy spokeswoman said.
"We will not intervene with this process."
But a spokesman for Mr Jaafari said the prime minister had heard of the message.
"How can they do this?" Haidar al-Ubaidi asked.
"An ambassador telling a sovereign country what to do is unacceptable," he added.
"The perception is very strong among certain Shia parties that the US, led by Mr Khalilzad, is trying to unseat Mr Jaafari."