A 2,500-strong U.S. force, backed by tanks and artillery, massed Tuesday on the outskirts of Najaf (search) for a showdown with a radical cleric, raising fears of an assault on the holiest Shiite city. In besieged Fallujah (search), two intense battles killed a Marine, forced down a U.S. helicopter and severely strained a truce.

Meanwhile, a State Department official said four bodies have been found in Iraq. The bodies may be those of private contractors missing since an assault on their convoy outside Baghdad amid a wave of kidnappings of at least 22 foreigners.

With the killings of the Marine and a soldier Tuesday, April became the deadliest month for the U.S. military since the Iraq war began in March 2003. At least 83 troops have been reported killed in action in less than two weeks. Last November, 82 were killed.

In the south, Iraqi politicians and ayatollahs tried to negotiate a solution to avert a U.S. attack on Najaf, which would outrage the nation's relatively pro-U.S. Shiite majority and could turn what has been a limited revolt by a single militia into an outright Shiite rebellion. A military advance could also inflame Shiites in neighboring Iran.

The vehemently anti-U.S. cleric, Muqtada al-Sadr (search), was holed up in his office in Najaf, shielded not only by gunmen but by the presence of the city's main shrine only yards away. He vowed to continue what he called "a popular revolution" to end the U.S. occupation of Iraq.

"I fear only God. I am ready to sacrifice my blood for this country. But I call on the Iraqi people not to let my killing put an end to their rejection of the (U.S.) occupation," al-Sadr told Lebanon's Al Manar television station.

At a news conference Tuesday, President Bush said "al-Sadr must answer the charges against him and disband his illegal militia." He added that "members of the Governing Council are seeking to resolve the situation in the south."

U.S. commanders vowed to kill or capture al-Sadr, though officials suggested they would give negotiations a chance.

"The target is not Najaf. The target is Muqtada al-Sadr and his militia," said Brig. Mark Kimmitt, deputy head of U.S. military operations in Iraq. "We will hunt him down and destroy him. We would prefer it not in Najaf or Karbala. We have very great respect for the shrines, for the Shiites."

The U.S. military has been fighting on several fronts across Iraq this month — against al-Sadr's militia in the south, against Sunni insurgents in the central city of Fallujah, as well as increased violence in Baghdad and elsewhere.

In Fallujah, a truce called by both sides since Sunday — three days after the Marines called a unilateral halt — was strained by new fighting. One Marine was killed by mortar fire.

A force of 20 insurgents attacked a Marine position in a residential neighborhood, then damaged an armored vehicle that came to support it, Capt. James Edge said.

A fierce battle followed to extract the vehicle as F-15s overhead fired on gunmen. Early Wednesday, A-130 gunships fired on a row of buildings in the area from which ambushes have repeatedly been launched, Edge said.

Outside the city, an MH-53 Pave Low helicopter — used to ferry special operations soldiers and large enough to carry 38 troops plus a crew of six — was hit by ground fire early Tuesday. A Marine team that came to secure it was ambushed and suffered casualties.

Edge said the two ambushes Tuesday were the best organized and largest operations seen by the guerrillas in recent days.

About 880 Iraqis have been killed this month, according to an Associated Press count based on statements by Iraqi hospital officials, U.S. military statements and Iraqi police. Among those are more than 600 Iraqis — mostly civilians — killed in Fallujah, according to the city hospital's director.

The State Department official confirmed the discovery of the four bodies — a possible new development in the kidnappings that began last week. The identities and nationalities of the victims were unknown, said the official, speaking only on condition of anonymity.

The U.S. contractor Halliburton said it did not know whether the bodies were those of any of its seven civilian employees missing since their convoy was attacked by gunmen in the Abu Ghraib district west of Baghdad. Two U.S. soldiers are also missing since that clash.

One of the seven missing employees — Thomas Hamill, a 43-year-old truck driver — is known to have been abducted. His captors have threatened to kill and mutilate him unless U.S. troops ended their assault on Fallujah. The deadline passed Sunday with no word on his fate.

NBC News reported that the four bodies were in a shallow grave between Fallujah and Abu Ghraib, scene of the convoy attack. U.S. officials were led to the grave by an Iraqi. The remains had not been identified, the State Department official said. He had no further information on the discovery.

Meanwhile Tuesday, a French journalist and four Italians working as private guards were also reported abducted.

The kidnappings have chilled foreign aid, media and contracting agencies working in Iraq. An AP tally shows that 22 were being held, including three Japanese whose captors have threatened to kill them. At least 35 others had been taken hostage and released. Nine Americans, including two soldiers, were missing.

Dan Senor, the spokesman for the U.S.-led administration, said Tuesday that 40 foreign hostages from 12 countries were being held by Iraqi insurgents. He said the FBI is investigating.

Skirmishes have begun in the Najaf campaign, where the U.S. force is even larger than the Marine force besieging Fallujah. Overnight, gunmen and roadside bombs ambushed the 80-vehicle convoy of troops heading to Najaf, killing one soldier and wounding two others and an American contractor.

After the force deployed outside Najaf, a U.S. unit pursued a group of al-Sadr militiamen into the city and killed a number of them, officers said.

Al-Sadr, a 30-year-old cleric, made a rare outdoor appearance Tuesday, returning to his office from prayers at the nearby Imam Ali Shrine, the city's main holy site. His office is less than a stone's throw from the shrine — meaning any U.S. assault against him there could be explosive if the shrine is damaged.

The area around the shrine is so religiously sensitive that coalition troops don't enter it even with peaceful intentions.

"We've got to get this right," said Col. Dana J. H. Pittard, commander of forces outside Najaf. If not, it will anger "the whole Muslim world between Morocco and Indonesia."

He said there are believed to be about 1,500 active al-Sadr supporters in Najaf. Al-Sadr's al-Mahdi Army militia launched an unexpectedly powerful uprising on April 4, battling U.S. troops in Baghdad — killing eight — and coalition forces in the south.

In Najaf, Iraqi leaders launched hurried negotiations over the weekend, apparently at the instigation of Iraq's most powerful Shiite cleric, Grand Ayatollah Ali al-Husseini al-Sistani. A mediator met Tuesday for the second day with al-Sadr's representatives.

Al-Sistani's son and the sons of two other grand ayatollahs met al-Sadr Monday night and assured him of their opposition to any U.S. strike on Najaf.

The grand ayatollahs — older, moderate leaders with immense influence among Shiites — have long kept al-Sadr at arm's length. The meeting reflected their eagerness to avoid bloodshed and al-Sadr's increasing influence.

U.S. officials were not participating directly in any negotiations with al-Sadr, but they appeared to be giving the talks a chance.