Drafters of a new constitution have summoned leaders of Iraqi religious, ethnic and political blocs to an emergency national summit this week in an attempt to hash out the toughest, deadlocking issues in reshaping the country, U.S. and Iraqi officials said Tuesday.

U.S. officials in Washington said the gathering will take place Thursday and Friday, if enough leaders -- particularly members of Iraq's Sunni Arab minority -- agree.

Humam Hamoudi, chairman of the constitution-writing committee, has called for the forum, and U.S. officials said the aim is to bring in leaders from outside the committee, particularly Sunni Arabs. The U.S. officials spoke on condition of anonymity because of the sensitivity of the talks.

Sunni Arab delegates returned to the committee meetings Tuesday after a week-long boycott. But it remained clear that the major divisive issue was federalism, which Sunni participants charge would break up the country.

In ongoing insurgent violence, meanwhile, attackers hurled grenades and fired automatic weapons at a bus carrying workers near Baghdad's Abu Ghraib prison, killing 18 of the laborers, police said.

The U.S. military said Tuesday that four U.S. soldiers were killed Sunday night when their vehicle ran over a roadside bomb in the southwest of the city. Jim Driscoll, a spokesman for the Georgia National Guard, said the victims were assigned to the 48th Infantry Brigade, the Associated Press reported. He said they were the Georgia Guard unit's first combat casualties since World War II.

Separately, a video posted on the Internet showed an Algerian diplomat who was kidnapped last week. The diplomat, wearing a blindfold, and another Algerian seized with him had been subjected to "the judgment of God" and would be killed, said an accompanying statement, attributed to an insurgent group led by Abu Musab Zarqawi.

Progress toward an Aug. 15 deadline for completing a constitutional draft is seen as essential by the interim government and its U.S. backers to demonstrate that political reform has momentum over the relentless insurgency.

A credible draft is considered equally vital to convince the Sunni Arab minority that there has been progress in a country also racked by common crime and chronic shortages. Many insurgent fighters are recruited among disaffected Sunnis.

The draft is expected to be delivered to the National Assembly shortly after the Aug. 15 deadline. A national referendum on the constitution is set for October.

The attempt to draw new participants was intended to give additional impetus to the process, officials said. "There is a notion that they can get it done by August 15, so there is a big push to have big leaders involved so there is no posturing inside the constitutional committee to make life difficult," said a senior Iraqi official familiar with the talks.

Leaders of "all groups and political parties and the groups which are represented in the National Assembly" should participate in the constitutional summit this week, said Kamran Qaradaghi, spokesman for Iraqi President Jalal Talabani. Talabani would likely be among the participants, Qaradaghi said.

In Washington, a State Department official said Massoud Barzani, the Kurdistan regional president and political leader, who seldom leaves his northern stronghold, "is coming down from the mountains" for the meeting in Baghdad.

"It's the reality of any political system that political leaders need to be involved in deal-making, either directly or indirectly," said a senior State Department official, describing the meeting as a positive development. "This is bringing them into the process more directly."

Committee delegates on Tuesday established an 18-member subcommittee assigned to work out major unresolved issues, chiefly federalism, said Saad Jawad Qandeel, a Shiite Muslim committee member.

Kurds are pushing for federalism as the only acceptable system for their already autonomous north. Some in the Shiite majority also want the same rights for the heavily Shiite south. Sunnis, who governed until the 2003 U.S. invasion that ousted President Saddam Hussein, reject such a provision.

Sunni delegates said Tuesday that they flatly reject a southern Shiite federal region under any circumstance. They proposed putting off the federal question until some months after the constitution is approved, said Salih Mutlaq, a leading Sunni on the committee.

But Qandeel said that position was unacceptable to Shiites. He suggested that a federal system might become part of the constitution despite Sunni objections. Qandeel said consensus was not necessarily the same as unanimity.

Other undecided basic questions include the role of religious law in the constitution, the issue of women's rights and the official name of the country. Khudaier Kuzaie, a committee member, said Tuesday that the latest draft he had seen cited Islam as one main source of laws, rather than the only source -- a key point in determining the extent of religious influence over the government.

A Western diplomat on Friday said the latest draft had dropped a provision, objected to by some women's groups, that said decisions related to divorce, inheritance and other family matters would be governed by sharia, or Islamic law.

Diplomats and delegates have stressed that numerous drafts of the constitution are circulating, with the proposals changing daily.

Wright reported from Washington. Special correspondents Omar Fekeiki, Naseer Nouri and Khalid Saffar contributed to this report.