Insurgents attacked an annual pilgrimage by Shiite Muslims in northern Baghdad (search) on Wednesday, killing three people and wounding dozens in a mortar attack. The town of Qaim (search) near the Syrian border was reported to be deserted and quiet after a day of clashes between rival tribes and air strikes by U.S. jets.

The rocket and mortar attack in Baghdad occurred as hundreds of thousands of people gathered at the Imam Mousa al-Kadim shrine in the capital's Kazimiyah (search) district for the annual commemoration the death of the Shiite saint.

Four mortar rounds slammed into the crowd, killing three and wounding at least 35, police Maj. Falah Al-Mohammdawi said. A military statement said U.S. Apache helicopters fired on the attackers after observing the rocket launches.

Television reports said about one million pilgrims from many parts of Baghdad and from outlying provinces had gathered near the shrine. In Saadoun Street, a large tent was erected where volunteers distributed water, juices and food to the those arriving from outside the capital.

The attack came a day after the latest twist in Iraq's constitutional saga. On Tuesday, U.S. ambassador Zalmay Khalilzad (search) raised the possibility of further changes to the draft charter finalized by the dominant Kurdish and Shiite Arab bloc but vehemently opposed by Arab Sunnis who form the core of the armed insurgency.

Sunnis had demanded revisions in the constitution, and Khalilzad's move indicated that President Bush's administration has not given up its campaign to obtain some sort of Sunni endorsement for the national charter.

Khalilzad said he believed "a final, final draft has not yet been, or the edits have not been, presented yet" — a strong hint to Shiites and Kurds that Washington wants another bid to accommodate the Sunnis.

Shiite leaders had no comment on the ambassador's remarks. As constitution wrangling drew to a close last week, Shiite officials complained privately that the Sunnis were stonewalling and that further negotiations were pointless.

Khaled al-Attiyah (search), a Shiite member of the constitution drafting committee, insisted Tuesday that "no changes are allowed" to the draft "except for minor edits for the language."

This indicated that the Shiites and Kurds would be unlikely to compromise on their core demand for Iraq to be turned into a loose federation. Sunnis fear this would eventually lead to the breakup of the nation, which has been ruled as a centralized entity since it was established by British occupiers in the 1920s.

Sunni Arabs form an estimated 20 percent of the population. They could still scuttle the charter because of a rule that states that if two-thirds of the voters in any three provinces reject the draft, it would be defeated. The Sunnis are dominant in four of Iraq's 18 provinces.

Even if the Sunnis lose the referendum, a bitter political battle at a time when the Sunni-led insurgency shows no sign of abating could plunge the country into a full-scale sectarian conflict.

Meanwhile, eyewitnesses said the town of Qaim, 200 miles northwest of Baghdad, was quiet and virtually deserted Wednesday after a day of heavy fighting between the pro-government Bumahl tribe and the pro-insurgent Karabilah tribe. Iraqi officials said 45 people had died in the clashes, during which hundreds of residents fled their homes and took refuge in the surrounding countryside.

The border region is considered a prime infiltration route for smugglers and foreign militants trying to reach central and western Iraq.

The U.S. military said jets bombed the region around Qaim and destroyed houses used by "a known terrorist."

The Shiite Supreme Council for the Islamic Revolution (search) in Iraq condemned attacks by foreign fighters against "our beloved people" and urged the government to "stop criminals and terrorists from crossing into Iraq."