Senior Shiite politicians said today that the American ambassador has told Shiite officials to inform the Iraqi prime minister that President Bush does not want him to remain the country's leader in the next government.

Prime Minister Ibrahim al-Jaafari holding a news conference last week in Baghdad, Iraq. It is the first time the Americans have directly intervened in the furious debate over the country's top job, the politicians said, and it is inflaming tensions between the Americans and some Shiite leaders.

The ambassador, Zalmay Khalilzad, told the head of the main Shiite political bloc at a meeting last Saturday to pass a "personal message from President Bush" on to the prime minister, Ibrahim al-Jaafari, who the Shiites insist should stay in his post for four more years, said Redha Jowad Taki, a Shiite politician and member of Parliament who was at the meeting.

Ambassador Khalilzad said that President Bush "doesn't want, doesn't support, doesn't accept" Mr. Jaafari to be the next prime minister, according to Mr. Taki, a senior aide to Abdul-Aziz al-Hakim, the head of the Shiite bloc. It was the first "clear and direct message" from the Americans on the issue of the candidate for prime minister, Mr. Taki said.