NAJAF, Iraq - U.S. tanks rolled into the Old City of Najaf toward a holy Shiite shrine where militants were hiding today as participants at a national conference voted to send a delegation here to try to negotiate an end to the fighting.

The city, which had been quiet early this morning, was hit by series of explosions in the late morning that shook the vast cemetery, the scene of many battles between U.S. forces and militants. Witnesses also reported U.S. tanks had moved to within 500 yards of the revered Imam Ali Shrine.

"We are proceeding with our operations. We are moving forward and we captured some positions inside the Old City from the south during the night and this morning," Police Chief Brig. Ghalib al-Jazaari said.

Fighting resumed Sunday after negotiations and a cease-fire collapsed. Two U.S. soldiers were killed in Najaf fighting Sunday, and a Marine was killed in Iraq's western, largely Sunni province of Anbar. At least 934 U.S. service members have been killed in Iraq since March 2003.

Also today, officials reported that a French-American journalist and his Iraqi translator have disappeared in the southern city of Nasiriyah.

The journalist, Micah Garen, and his translator Amir Doushi went missing while walking through a busy market in the city, said Adnan al-Shoraify, deputy governor of Dhi Qar province. He said the translator's family had first reported the two missing.

The Arab television station Al-Jazeera said the journalist had been kidnapped and provided no other details. Al-Shoraify could not confirm whether Garen, 33, was abducted.

The fighting in Najaf has cast a pall over the National Conference in Baghdad, an unprecedented gathering of 1,300 religious, tribal and political leaders from across Iraq meant to be a key first step toward democracy.

Some of the delegates threatened to walk out unless the crisis was resolved. The conference today voted to send a delegation to Najaf to ask radical Shiite cleric Muqtada al-Sadr to tell his followers to drop their weapons and join the country's political process.

"The door is very open to all Iraqis, regardless of their religion, ethnic background, to join the free political process," Shiite cleric Hussein al-Sadr, a distant relative of Muqtada al-Sadr, told the conference.

Muqtada al-Sadr's aides said they supported efforts to end the violence.

"We are ready to accept any mediation for a peaceful solution," al-Sadr aide Ahmed al-Shaibany said.

At the same time, however, al-Shaibany called on tribal chiefs throughout Iraq to travel to Najaf to form human shields to protect al-Sadr's Mahdi Army militants and the Imam Ali Shrine.

Fighting on Sunday apparently caused minor damage to the outer wall of the shrine compound, ripping off some tiles and leaving some holes.

With Sunday's deaths, at least eight U.S. troops have been killed in Najaf, along with about 20 Iraqi officers, since fighting there began Aug. 5. The U.S. military estimates hundreds of insurgents have been killed, but the militants dispute the figure.

In other violence, two civilians were killed and four others injured in the city of Baqouba today when a mortar hit their house, said Ali Hussein, a medic at the main hospital in Baqouba.

It was not known who fired the mortar, but insurgents frequently clash with U.S. troops and Iraqi security forces in the city, 35 miles northeast of Baghdad.

A roadside bomb in Baqouba also wounded three members of the Iraqi National Guard.

In the volatile Sunni city of Fallujah, west of Baghdad, U.S. warplanes bombed three neighborhoods Sunday afternoon, killing five civilians and wounding six others, said Dr. Adil Khamis, of Fallujah General Hospital.

The three-day conference, which started Sunday, aims to give a broad spectrum of Iraqis a voice in the political process and increase the legitimacy of interim Prime Minister Ayad Allawi's interim government.

Just hours after the heavily guarded meeting began, however, insurgents fired a mortar barrage that landed at a nearby commuter bus station, killing two people and wounding 17 others.