U.S. soldiers opened fire on a truck packed with explosives Saturday, killing the driver, and three Americans were wounded when the truck crashed on a bridge and exploded.

The apparent vehicle-bomb attack was in Habaniyah (search), west of Baghdad. In Amarah (search), seven British soldiers were wounded in a three-hour firefight with unknown attackers in southern Iraq, coalition officials said. Three Iraqis were killed, British officials said.

Meanwhile, Shiite members of Iraq's Governing Council (search) conferred with the country's top Shiite cleric, Grand Ayatollah Ali al-Husseini al-Sistani, to resolve a dispute that held up the signing of an interim constitution.

Al-Sistani rejected two clauses in the interim charter — one that would have given Iraq's Kurds the power to scuttle a permanent charter and another that would have provided for a single president instead of a rotating leadership.

Reflecting al-Sistani's objections, the Shiite council members refused on Friday to sign the interim constitution hours before it was supposed to be signed, embarrassing U.S. officials and providing a stark reminder of the ayatollah's enormous influence in Iraqi politics.

With negotiations reopened, a Kurdish official said his side would not consent to changing the clause, which was agreed to by the entire council when it approved the constitution on Monday after several days of intense debate.

"We are sticking to it because it's a legitimate demand," said Kosrat Rasul, an official in the Patriotic Union of Kurdistan, one of two main Kurdish parties on the council.

Officials would not comment on the talks among the Shiites in Najaf, and it was not clear whether the Shiite politicians were trying to work out alternatives with al-Sistani, explore what phrasing would be acceptable for him or persuade him to drop his objections.

Still, they said they expected to resolve the dispute by Monday.

"We have announced that Monday is the date for the signing of the law and we are determined to stick to this date," Mohammad Bahr al-Ulloum told reporters in Najaf, site of Shiites' holiest shrine.

The interim constitution, which will remain in effect until the end of 2005 after a permanent charter is approved, is a crucial part of a U.S. plan for handing over power to the Iraqis on June 30. It took intense negotiations last weekend, shepherded by the Americans, to overcome sharp divisions and reach a deal.

Al-Sistani's son, Mohammed, shuttled back and forth between his father's home and Bahr al-Ulloum's office in Najaf, where the Shiite council members gathered Saturday.

Whatever compromise is worked out with al-Sistani must be agreed to by the other 20 members of the council.

"They should have discussed this issue since the beginning. It was a surprise for everyone," Rasul, of the PUK, told AP. "Everybody was prepared to sign the constitution" on Friday.

Mahmoud Othman, an independent Kurd on the council, said he was hopeful the document would be signed Monday. He invited al-Sistani to send a delegate to talks in Baghdad to ensure a deal is reached.

"If Sistani wants to a send a representative to the council, he can," he told Associated Press Television News on Saturday.

Al-Sistani has twice before derailed U.S. plans, with objections to the timetable and methods for transferring sovereignty to an Iraqi government. The Bush administration wants to carry out the transfer well before November U.S. presidential elections.

The Shiites opposed a clause that Kurds got into the charter concerning a referendum planned for next year to approve the permanent constitution. The clause says that even if a majority of Iraqis support the permanent constitution, the referendum would fail if two-thirds of the voters in three provinces reject it.

The Kurds control three provinces in the north, allowing them to stop any constitution that encroaches on their self-rule. Al-Sistani objected to a minority having the power to block any charter approved by the Shiite majority.

Several officials said another cause of dispute was the makeup of the presidency. The draft approved earlier in the week set up a single president with two deputies.

The Shiites revived their demand for a presidency that would rotate among three Shiites, a Kurd and a Sunni — giving the Shiites a dominant role. U.S. and some Iraqi officials, however, said the shape of the presidency was not in dispute.