U.S. warplanes bombed alleged safe houses being used by Abu Musab Zarqawi's insurgent group near the Syrian border Tuesday during what one local leader called an unprecedented push by a Sunni Arab tribe to drive out Zarqawi's foreign-led forces.

The bombings occurred along the Euphrates River in two towns that U.S. officials and Iraqis describe as havens and transit points for insurgents moving weapons, money and recruits into Iraq from Syria. Ali Rawi, an emergency room director in the border city of Qaim, said at least 56 people -- the majority of them apparently followers of Zarqawi -- were killed in Tuesday's airstrikes and ground fighting. Zarqawi's group, al Qaeda in Iraq, said in a statement posted in local mosques that it had lost 17 men.

Neither U.S. nor Iraqi officials gave death tolls.

The clashes between Sunni Arab tribes and insurgents, coupled with growing vows by Iraq's Sunni minority to turn out in force for national voting in the coming months, coincided with U.S. hopes for defusing the two-year-old insurgency. U.S. military leaders have repeatedly expressed optimism that public anger at insurgent violence would deprive insurgents of their base of support.

A tribal leader near the Syrian border, Muhammed Mahallawi, said his Albu Mahal tribe began the latest fighting against Zarqawi's insurgents after they kidnapped and killed 31 members of his tribe to punish them for joining the Iraqi security forces.

"We decided either we force them out of the city or we kill them," with the support of U.S. bombing, Mahallawi said by telephone.

Sunni Arab tribes in the western province of Anbar have clashed sporadically with Zarqawi's organization since at least May, usually in revenge for killings of tribe members accused of collaborating with U.S. forces or the Iraqi government. This month in Ramadi, the capital of Anbar, tribes took up arms to block Zarqawi's group from enforcing his ultimatum for all Shiite Muslim families to leave the city. Fighting there killed several fighters on both sides.

Local officials said Tuesday that Mahallawi's tribe and the insurgents had been fighting near the border for at least three days. Rawi, the emergency room director, said at least 61 people had been killed since the fighting began. The majority of the dead Tuesday were in the Western-style clothes and athletic shoes often worn by Zarqawi's fighters, Rawi said.

The U.S. military confirmed six airstrikes at dawn Tuesday on two residences in and around Husaybah believed to house insurgents. When survivors of those attacks drove three miles to another residence in Karabilah, the U.S. warplanes hit that house with two bombs, a U.S. military statement said.

The military said it believed the precision-guided bombs killed several insurgents.

Residents said one of the airstrikes hit a weapons cache, setting off explosions in the house. Another targeted building was a former clinic that had been taken over by Zarqawi, residents said.

There was no word from the U.S. military on whether the airstrikes were coordinated with Zarqawi's tribal opponents. On Friday, a U.S. military statement credited strikes by Marine F-18D fighters on an alleged Zarqawi safe house in Husaybah to tips by telephone from local citizens. With Zarqawi and his allies trying to consolidate control of the border towns, "local leaders and sheikhs are resisting AQIZ's murder and intimidation campaign," a military statement said, using an acronym for Zarqawi.

Mahallawi said his tribe had asked local residents not to aid or house Zarqawi's fighters. Some of the people refused the request, he said.

The fighting comes as Iraq's Sunni Arabs are registering to vote in large numbers -- a sharp turnaround from January, when threats by insurgents and calls for boycotts led Sunnis to largely stay out of elections that seated Iraq's National Assembly.

Iraqis are scheduled to cast ballots by Oct. 15 on the new constitution drafted by the National Assembly. If the charter passes, Iraqis would vote again Dec. 15 to elect a full-term government.

The draft constitution released this week by the Shiite- and Kurdish-led transitional government has angered many Sunnis by opening the way for creation of a Shiite-populated region in the oil-rich south under a loose federal government. Many Sunnis see that proposal, along with the constitution's formalization of existing Kurdish self-rule in the north, as the start of the breakup of Iraq.

In Baghdad, a Sunni Arab critic of the draft appeared alongside U.S. Ambassador Zalmay Khalilzad to say Sunnis should resolve their objections peacefully. "We believe the best way to solve problems is through elections," Adnan Salman Dulaimi told reporters at a news conference.

Dulaimi pledged to work to defeat the draft charter in the October vote.

"On federalism, we reject it because it will lead to tearing up the country," Dulaimi said. "We call on all Iraqis for unity, solidarity, closing of ranks to confront those who want to undermine the unity of Iraq. No to sectarianism, no to federalism."

Dulaimi also accused the forces of Iraq's Shiite-led Interior Ministry of carrying out political killings, including those of three dozen Sunni men whose bodies were found last week, with bullet wounds, in a dry riverbed southeast of Baghdad. "Who could have kidnapped them and reached that area without being stopped by checkpoints or police patrols?" he asked.

U.S. and Interior Ministry officials variously deny political killings by the ministry or say investigations are underway. No results of investigations have been announced.

Khalilzad suggested at the same news conference that constitutional negotiations, which concluded Sunday after months of deal-making, could reopen, apparently in answer to Sunni objections.

"I believe that a final, final draft has not yet been, or the edits have not been, presented yet, so that is something that Iraqis will have to talk to each other and decide for themselves," Khalilzad told reporters.

Salih Mutlak, a member of the constitution-writing committee and the most vocal Sunni critic of the draft, said he would welcome the reopening of negotiations. But neither Mutlak nor others involved in the talks said they knew of any resumption, which would likely spur strong objections from Shiite leaders and others.

Hundreds of Sunnis rallied outside Ramadi on Tuesday to denounce the proposed constitution, the Associated Press reported. Protesters carried posters of Saddam Hussein, who has reemerged in recent days as a symbol of Sunni anger over the constitution. "No to federalism, no to dividing Iraq," the slogan on one banner declared.

Political violence Tuesday included the fatal shootings of two police colonels in Baghdad and in Kirkuk and a suicide car-bombing that killed two police officers in the northern city of Samarra.

Meanwhile, Reporters Without Borders said an Iraqi television journalist had been killed, according to the Associated Press. Rafed Mahmoud Rubai was shot Saturday by unidentified gunmen while covering a demonstration east of Baghdad. He became the 67th journalist to die in the Iraq war.

In the Shiite holy city of Najaf, government security forces late Tuesday announced that entrances to the town had been closed, apparently to block a religious pilgrimage announced by Moqtada Sadr, a popular Shiite cleric.

Special correspondent Saad Sarhan in Najaf contributed to this report.

Adnan Salman Dulaimi, a Sunni, addresses reporters in Baghdad at a news conference with U.S. Ambassador Zalmay Khalilzad.