Najaf, Iraq ? Negotiations to end the fighting in Najaf broke down Saturday, threatening to spark a resurgence of the fierce clashes between Shiite militants and a combined U.S.-Iraqi force that have plagued this holy city for more than a week.

The collapse of talks will likely cast a pall over Iraq’s National Conference, which starts today, gathering 1,300 delegates from all over Iraq in what is considered a vital step toward establishing democracy.

The chief government negotiator said he decided to quit the talks in Najaf after three fruitless days, but representatives of militant cleric Muqtada al-Sadr said a deal had been all but reached before interim Prime Minister Ayad Allawi personally intervened to quash it.

“It is a conspiracy to commit a big massacre,” al-Sadr’s top negotiator, Sheik Ali Smeisim, told the pan-Arab Al-Jazeera television station.

Soon after the talks broke down, a massive Army and Marine force of tanks, Humvees and armored vehicles lined up inside a U.S. military base in Najaf for an assault on the militants, which Allawi reportedly called off.

“We were sitting here waiting for authorization to go clear the militia. We never got that authorization,” said Marine Maj. David Holahan. “We’ll continue operations as the prime minister … sees fit.”

U.S. forces called a halt to a major offensive in the city on Friday to give negotiations a chance. The fighting in Najaf has angered many in Iraq’s Shiite majority, complicating a difficult situation for Allawi’s U.S.-backed government, which has been keen to show it is in control.

The chairman of the National Conference, Fuad Masoum, insisted the violence would not affect the three-day gathering.

“This is a perfect time for the conference to discuss the current problems and find solutions,” he said.

About 10,000 demonstrators from as far away as Baghdad arrived in Najaf on Saturday to show their solidarity with the militants and act as human shields to protect the city and the holy Imam Ali shrine, where fighters from al-Sadr’s Mahdi Army have taken refuge since the fighting started Aug. 5.

Amid worries that fighting would resume, coalition officials reiterated Saturday they would not enter the shrine.

“It is not our intention to go anywhere near the holy sites. We understand their significance to the Shia, and we respect the Shia,” Maj. Gen. Andrew Graham, deputy commanding general of the Multination Corps, told The Associated Press.