BAGHDAD, Iraq, March 28 — 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.

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.

An American Embassy spokeswoman confirmed that Mr. Khalilzad and Mr. Hakim had met, but declined to comment directly on what they had spoken about.

The Americans have harshly criticized the Jaafari government in recent months for supporting Shiite militias that have been fomenting sectarian violence and pushing Iraq closer to full-scale civil war. Ambassador Khalilzad has sharpened his attacks in the last week, saying the militias are now killing more people than the Sunni-led insurgency.

There is growing concern among American officials that Mr. Jaafari is incapable of reining in the private armies, especially since Moktada al-Sadr, the anti-American cleric who leads the most volatile of the militias, is Mr. Jaafari's strongest backer.

Haider al-Ubady, a spokesman for Mr. Jaafari, said the prime minister had heard of the ambassador's verbal message through officials in his party, and accused the Americans of trying to subvert Iraqi sovereignty and weaken the Shiite ranks.

"How can they do this?" Mr. Ubady said. "An ambassador telling a sovereign country what to do is unacceptable."

"The perception is very strong among certain Shia parties that the U.S., led by Khalilzad, is trying to unseat Jaafari," he added.

The American Embassy spokeswoman, Elizabeth Colton, confirmed that the ambassador did see Mr. Hakim on Saturday. The two meet regularly to discuss Iraq's political situation.

"The decisions about the choice of the prime minister are entirely up to the Iraqis," Ms. Colton said. "This will be an Iraqi decision."

President Bush, in general comments about Iraq, said at the White House that he was pleased the Iraqis were "continuing to discuss who will fill the key slots in the government."

Tensions between Shiite leaders and the American government, which had been rising for months, have boiled over following an assault Sunday night by American and Iraqi forces on a Shiite mosque in northern Baghdad. Shiite leaders say at least 17 civilians were killed in the battle, mostly political party members, while American commanders say the soldiers had fought insurgents.

The reported pressure from the American government over Mr. Jaafari's nomination is another sign of the White House's acute impatience over the deadlocked talks to form a four-year government. The nomination has become one of the most contentious issues in those talks, with the main Kurdish, Sunni Arab and secular blocs calling for the Shiites to replace Mr. Jaafari. American officials say the chronic delay in installing a government has created a power vacuum where lawlessness is thriving and a low-level civil conflict is raging.

Shiite leaders on Monday suspended their participation in the government negotiations, saying they were enraged by the mosque assault.

At least 21 people were abducted in four separate incidents in Baghdad today, in the biggest wave of kidnappings in a month, an Interior Ministry official and a hospital guard said. In one incident, 15 men in Iraqi Army uniforms dragged at least six people from a money exchange shop and stole nearly $60,000 of currency. The other cases involved people dressed as Interior Ministry commandos snatching people from two electronics shops, and criminals abducting two hospital workers.

Last month, gunmen in commando uniforms kidnapped 35 people from the offices of an Iraqi security company. A handful have been released.

Also today, the police discovered 14 bodies in western Baghdad, all executed with gunshot wounds to the head, apparently the latest victims of sectarian bloodletting. Iraqi forces on Monday found 18 bodies near Baquba of people who had been killed in a similar manner. Earlier reports of 30 beheaded bodies discovered in the same area were wrong, the Interior Ministry official said.

The Iraqi national security minister, Abdul Karim al-Enizi, said on the state-run Iraqiya network tonight that the Iraqi forces who had raided the Shiite mosque in Baghdad were not part of the Interior or Defense Ministries. A survivor said the soldiers did not speak Arabic well, implying they may have been Kurdish militiamen working with Americans, Mr. Enizi said.

The surging violence has shaken confidence in Mr. Jaafari. Among Iraqis, he has come under widespread criticism for failing to smash the Sunni-led insurgency, letting Shiite death squads run rampant and doing little to improve reconstruction.

Mr. Jaafari won the nomination by one vote in a secret ballot last month among the 130 parliamentarians of the main Shiite bloc. Mr. Hakim, the bloc's leader, had put one of his deputies, Adel Abdul Mahdi, up for nomination. But Mr. Jaafari, believed to be a favorite of Iran, won with the support of Moktada al-Sadr, the radical anti-American cleric who controls at least 32 parliamentary seats.

Before the parliamentary elections last December, Mr. Mahdi, one of two vice-presidents, was talked about as a possible American favorite for the post of prime minister. A Maoist turned Islamist and free-market advocate, he met with White House officials in Washington last fall. But Americans still harbor some suspicions: Like all leaders in his party, he has close ties to Tehran and to the Badr Organization, a powerful Iranian-trained militia.

He has said in the last week that he supports Mr. Jaafari as long as the Shiite bloc does too. Ambassador Khalilzad met with Mr. Mahdi today to try to patch up relations in the wake of the mosque assault.

The Constitution approved by voters last fall says the largest bloc in Parliament, in this case the Shiites, gets to nominate the prime minister. But a two-thirds vote of the 275-member Parliament is essentially needed to install the new government. So as long as the other major blocs remain opposed to Mr. Jaafari, the process is at a standstill.

In recent months, Ambassador Khalilzad has championed Sunni Arab inclusion in the next government while criticizing Shiite oversight of the current government, especially the security forces. As bodies pile up in the streets, believed to be the work of death squads, the ambassador has forcefully demanded that the Shiites disband their militias, especially the Mahdi Army, run by Mr. Sadr. All that has led to growing Shiite distrust of the Americans.

Last month, Mr. Hakim said the ambassador's anti-Shiite stand had contributed to the insurgent bombing of the golden-domed Askariya Shrine in Samarra, which houses the tombs of two Shiite imams. After the explosion, Shiite militiamen rampaged through eastern Baghdad, attacking Sunni mosques and killing hundreds.

Mr. Ubady, the prime minister's spokesman, said "it's been seen by the Shia that the post" of American ambassador, "which is now being held by Zalmay Khalilzad, is helping terrorists."

To break the deadlock over the prime ministerial nominee, some politicians opposing Mr. Jaafari have proposed that the Shiites offer up three candidates, including Mr. Jaafari, to be voted on by the entire Parliament. So far, the Shiites have scoffed at the idea and have banded together behind Mr. Jaafari. The question is whether there is dissent among the Shiites behind closed doors.

After Ambassador Khalilzad delivered his message last Saturday, Mr. Hakim called a meeting of senior Shiite politicians so they would all be aware of President Bush's views, said Mr. Taki, the aide to Mr. Hakim. Representatives of Mr. Jaafari attended that meeting.

Mr. Hakim and other Shiite leaders are visiting the holy city of Najaf to honor the anniversary of the Prophet Muhammad's death. Shiite participation in government negotiations will not resume until they return to Baghdad in a few days, Mr. Taki said. The next couple sessions will be occupied with discussions over the principals of the new government, he said, and only after those are settled will the politicians tackle the thorny topic of Mr. Jaafari.

"We don't have a clear answer about what to do," Mr. Taki said. "We need to discuss this."