Two weeks before Iraqis vote on a new constitution, with millions of copies already circulating for voters to study, U.S. Ambassador Zalmay Khalilzad is leading a drive for major changes in the charter to try to win crucial Sunni Arab support, according to Shiites, Sunnis and Kurds involved in the last-ditch negotiations.

Khalilzad in recent days agreed to take six Sunni demands to Shiite and Kurdish leaders for intensified negotiations. The demands for changes included some that Sunnis hope would keep political power and natural resources under the control of Iraq's traditionally strong central government, Nasser Janabi, a lead Sunni negotiator, said Monday.

"The six demands are our last suggestions," Janabi said. "We cannot give up any more rights. If they agree on these demands, the marginalized group will take another, positive position on the constitution."

The top U.S. officer in Iraq, Gen. George W. Casey, last week warned that if Sunni demands on the charter were frozen out, the Sunni-led insurgency could grow worse. Casey told reporters in Washington that the draft constitution as written jeopardized U.S. hopes for an early spring start to American troop withdrawals.

Khalilzad went to the Kurdish north late last week to urge charter changes on top Kurdish leaders, negotiators said. But neither Shiite nor Kurdish leaders have yet agreed to any of the changes, said both Shiite negotiator Ali Debagh and Kurdish negotiator Mahmoud Othman.

U.S. Embassy officials declined to comment on negotiators' account of the last-minute mediating by the American ambassador, who has pressed and cajoled political leaders for compromise on the charter since taking up his post this past summer.

Iraqis are due to vote on the new constitution Oct. 15. Promoters of the vote have prepared DVDs, pamphlets and forums to outline the charter to the public beforehand.

The draft constitution as written would allow Iraq's newly empowered Shiites and Kurds the option of transforming the nation into a loose federation, including a separate Kurdish north and potentially a separate Shiite south. Many Sunni Arabs, who had wielded tight control from Baghdad since Iraq's creation despite being a minority, said this would mean the breakup of the country.

Iraqi leaders pronounced the draft charter complete on Aug. 28, after three months of closed-door dealmaking and at least three missed deadlines. But Sunni negotiators condemned the draft, saying Kurds and Shiites had opted to try to ram the document through a national vote without their minority's support.

Khalilzad kept up behind-the-scenes negotiations even then to urge initially minor changes in hopes of bringing Sunnis on board. U.N. officials had to wait three more weeks before an official, final version was ready for a $2 million print run to let Iraqis see the draft charter before the vote. Iraqi bloggers joked that the constitution should be stored on a PowerPoint presentation to make changes easier.

Casey, speaking last week at a Pentagon news conference alongside Defense Secretary Donald H. Rumsfeld, said his prediction in July that substantial U.S. troop withdrawals could start in early spring had been based on the assumption of continued progress in security and stability in Iraq.

"Now this constitution has come out, and it didn't come out as the national compact that we thought it was going to be," Casey said. "And there's division there . . . and that caused the situation to change a little bit."

One of the Sunni demands for the charter would restore a dropped phrase that Iraq "should not be divided for any reason," Janabi said.

Two other demands stipulate that Iraq's oil and other resources should be under central management and that a decision on Iraq's future form of government should be made by a two-thirds majority of the next parliament. Sunnis largely stayed home during January elections of the current interim parliament but are expected to turn out in force for the next parliamentary elections in December.

A further Sunni demand would remove a phrase in the draft equating the former Baath Party to a terrorist group and would add a declaration that Iraq is "part of the Arab and Islamic nations." The final demand deals with conditions for citizenship.

Debagh said that Shiites would accept no changes on federalism but that they were considering at least four of the points.

"We have no objection to changes that help us to achieve mutual understanding, but there is no way that we would agree to change the basic principles of the constitution," he said.

In Najaf, the Shiite holy city, an aide to Moqtada Sadr said the influential Shiite cleric was monitoring the last-minute negotiations. Mustafa Yaqoubi said Sadr would wait until the week before the vote to announce his opinion on the charter, instructing his many followers how to vote.

Sadr is seen as sympathetic to the Sunni cause, although his statements regarding federalism have been guarded. His freeing of followers to vote as they wished helped make Shiite religious parties the biggest winners in the January vote.

Defeating the draft charter, under Iraq's interim constitution, would require a "no" vote by two-thirds majorities in at least three provinces. Sunnis have rallied to get out the "no" vote in the west and Baghdad, but the Shiite- and Kurdish-led interim parliament raised the bar by changing the requirement for defeat to a two-thirds "no" vote by registered voters.

The run-up to the negotiations is playing out during weeks of intensifying U.S. military action against insurgents in the west and growing insurgent attacks across the country.

Near the Syrian border, 1,000 U.S. troops kept up a three-day-old offensive that has killed at least three dozen suspected insurgents in Euphrates River towns dominated by foreign fighters and Iraqi insurgents, the U.S. military said.

The military on Monday reported the death of a U.S. soldier hit by indirect fire -- meaning either mortar rounds or rockets -- in the western city of Ramadi.

Insurgent attacks Monday included a roadside bomb in Baghdad that hit the convoy of Oil Minister Ibrahim Bahr Uloum. Three of his bodyguards were killed, authorities said.

Wright reported from Washington. Correspondent Ellen Knickmeyer and special correspondent Bassam Sebti in Baghdad and special correspondent Saad Sarhan in Najaf contributed to this report.

A Shiite member of Iraq's National Assembly visits an exhibition that supports the draft constitution, to be voted on in a referendum on Oct. 15. Many Sunni Arabs, so far dissatisfied by the draft, have rallied to get out the "no" vote. Zalmay Khalilzad, the American ambassador to Iraq, agreed in recent days to take six Sunni Arab demands to Shiite and Kurdish leaders for negotiation.