As outraged would-be voters protested at still not being shown copies of Iraq's proposed constitution, U.S. and Arab diplomats bore down on Sunnis, Shiites and Kurdish leaders Monday in Baghdad's fortified Green Zone to make last-ditch changes to the charter that would overcome Sunni opposition.

But meetings among political leaders -- including consultations with U.S. Ambassador Zalmay Khalilzad, a sit-down dinner to break the daily fast of the holy month of Ramadan and gatherings where Arab leaders exerted behind-the-scenes pressure -- all failed to reach a breakthrough, Iraqi officials in and close to the talks said.

With just five days until Iraqis are due to decide on the charter in a referendum, negotiators pointed to meetings Tuesday as the very last chance to haggle out a constitution that would hold Iraq together. Many Iraqis and U.S. officials fear the current draft will instead pull the country further apart. If voters approve a charter widely seen as shutting out the once-dominant Sunni minority, many expect a worsening of the strife that already has killed thousands since spring.

"It's very difficult to reach something that the Sunnis can agree to," Mahmoud Othman, a Kurdish official, said late Monday after the last talks broke up. "Tomorrow will be crucial."

Secretary of State Condoleezza Rice, en route to Central Asia, said the United States had actually been encouraged by the heavy debate on Iraq's controversial constitution. She predicted that debate among Iraq's diverse ethnic and religious factions would probably continue until the eleventh hour in an attempt to "give the Sunnis some confidence" that the constitution will represent their interests.

But she indicated that the United States was not deeply troubled about the possibility of a rejection, which would come if a two-thirds majority of voters in three or more of Iraq's 18 provinces oppose the document. "Whatever the decision, there are next steps. There will be elections in December," she told reporters traveling with her.

"The good news is that you're getting very heavy debate and interest in this entire process," Rice added, noting that Sunnis are registering to vote in large numbers, including some who disagree with the constitution.

The last-minute talks came as political violence Monday killed at least 18 people, including three Iraqi policemen, three Iraqi civilians and a U.S. soldier killed by a suicide car bomber outside the Green Zone. A convoy carrying Arab League diplomats was among the targets coming under fire in Baghdad, although no injuries were reported in that ambush.

Also Monday, officials said arrest warrants had been issued for the defense minister and 27 others from the government of former prime minister Ayad Allawi over the alleged disappearance or misappropriation of $1 billion in military procurement funds, the Associated Press reported. The officials include four other ministers from Allawi's U.S.-backed government, and many are believed to have left Iraq.

In Austria, printing presses are running off millions of ballots for Saturday's vote. Iraq's 15.5 million eligible voters are to say 'yes' or 'no' to a draft constitution that would change the country from a largely secular one with a strong central government to a loose federation of regions with a weak central government under heavy influence from Islamic law.

The constitution would formalize the autonomy that the Kurdish north has enjoyed since the Persian Gulf War. The charter would give the heavily Shiite south the option of splitting off as well, taking much of Iraq's oil revenue with it. Many Sunnis, who lost power with the 2003 routing of Saddam Hussein, strongly oppose that scenario, saying it would mean the breakup of Iraq.

While international organizations have helped print millions of copies of the draft for voters to study, many residents in the provinces and even in central neighborhoods of the capital say they have yet to get their hands on one of the blue-backed booklets with the proposed wording.

In Ramadi -- capital of the heavily Sunni province of Anbar, one of the centers of the insurgency -- residents accused the central government Monday of deliberately preventing them from seeing the proposed constitution.

"Let us read it so we can give our opinion!" said a slogan on one of the banners carried by hundreds of demonstrators. Another promised: "We will go to the referendum even if we do not see your constitution."

Sunnis generally say they will vote against the constitution. While the majority of Sunnis heeded boycott calls and insurgent warnings not to vote in January's national elections, Sunni leaders are calling on their people to enter the post-Hussein political process for the first time to try to vote down the charter. Even some militias in the Sunni-based insurgency have pledged to suspend attacks to allow Sunnis to turn out to defeat the charter.

Voter rejection would mean Iraq would elect another temporary assembly in December charged with drafting another constitution.

"We feel there is a wicked game that aims to block the Sunni from voting in the referendum by not distributing the constitution draft," said Omar Khalifa, a member of the Iraqi Islamic Party, a leading Sunni group that organized the protest in Ramadi.

Unsafe roads and attacks on government officials and election workers may have prevented distribution in the outlands. But even in Baghdad, comparatively few have seen the charter. "I haven't decided yet whether to vote or not," said Khalid Salim, a baker in central Baghdad. "I've read so little of it -- only what was published in the newspaper. Most of the people here do not understand it very well."

Other neighborhoods had copies of the draft distributed with monthly household ration baskets.

Excitement over the charter seems low, and officials' ongoing deal-making -- long after the Aug. 15 deadline for a draft and weeks after the transitional parliament approved a supposedly final version for a national vote -- has fostered a perception among many that the constitution will mean whatever politicians want it to mean.

Despite the complications of continued negotiations, Americans are pushing for compromises that could win Sunni support for the charter, in hopes of ending the insurgency through political means.

"Americans are working very hard to work out something," said Othman, the Kurdish official. Meetings on Monday for Khalilzad, the U.S. ambassador, included a session with Saleh Mutlak, one of the most unbending of the Sunni constitution negotiators.

Special correspondent Bassam Sebti in Baghdad and staff writer Robin Wright, traveling with Rice, contributed to this report.

Booklets explaining the draft constitution are handed out at a food rationing center in Baghdad even as heated negotiations continued over its content.