BAGHDAD, Iraq, AP -With U.S. mediation, Shiite Muslim and Kurdish officials negotiated with Sunni Arab leaders Sunday over possible last-minute additions to Iraq”s proposed constitution, trying to win Sunni support ahead of next weekend”s crucial referendum.

But the sides remained far apart over basic issues — including the federalism that Shiites and Kurds insist on, but that Sunnis fear will lead to the country”s eventual break-up. And copies of the constitution were already being passed out to the public.

Though major attacks in the insurgent campaign to disrupt the referendum have waned in recent days, violence killed 13 Iraqis Sunday.

In one attack, masked gunmen in police commando uniforms burst into a school in the northern town of Samarra, pulled a Shiite teacher out of his classroom and shot him dead in the hallway as students watched from their desks, police said. A suicide car bomb killed a woman and a child in the southern city of Basra.

A U.S. Marine was killed by a roadside bomb in the town of Ramadi, west of Baghdad, on Saturday, the military announced. It was the ninth American death during a series of offensives waged in western Iraq seeking to knock al-Qaida militants and other insurgents off balance and prevent attacks during Saturday”s national vote on the constitution.

The death brought to 1,954 the number of U.S. military personnel who have died since the beginning of the Iraq war in March 2003, according to an Associated Press count.

Interior Minister Bayan Jabr told an Arab newspaper it will take five years to put down Iraq”s insurgency. He said Iraq”s security forces only carry rifles &#34while the terrorists possess all kinds of advanced weapons.&#34

&#34Be patient with us for five years before restoring security and the state”s respect,&#34 Jabr said in an interview with the London-based Sharq al-Awsat newspaper.

Jabr said the number of foreign militants involved in Iraq”s insurgency had fallen to around 900, from as many as 3,000 three months ago.

Their ranks have fallen because of deaths inflicted by U.S. and Iraqi military offensives — but also because al-Qaida in Iraq has started sending fighters to other Arab nations to build terror networks there, Jabr told the newspaper.

As Sunni-led insurgents staged attacks to discourage Iraqis from voting in the referendum, the government launched a campaign to persuade Iraqis to go to the polls despite the threats — and despite calls by some Sunni Arab leaders for a boycott.

&#34We think (a boycott) would weaken Iraq because the only way that Iraq can recover is done by concentrating on the political process, writing the constitution and participating in it,&#34 government spokesman Laith Kubba said. &#34Any act that calls for violence or boycotting would deviate the country from its course.&#34

Many Sunni Arab leaders are calling on their followers to turn out in force to vote in the referendum — but to vote &#34no&#34 to defeat a draft they say will break Iraqi into pieces, with Shiite and Kurdish mini-states in the north and south and the Sunni minority left poor and weak in a central zone.

Though a minority, Sunnis can defeat the charter if they garner a two-thirds &#34no&#34 vote in any three of Iraq”s 18 provinces — and they have the potential to make that threshold in four provinces. Turnout is key, since they must outweigh Shiite and Kurdish populations in some of those areas.

Even with copies of the official text of the constitution being distributed to voters to consider before the polls, all sides were debating last-minute changes in a bid to swing some Sunnis to a &#34yes&#34 vote. Kurdish leader Massoud Barzani met with Sunni Arab leaders Saturday and Sunday trying to convince them on the changes, officials from all sides said.

U.S. Ambassador Zalmay Khalilzad &#34has a central role in the talks,&#34 said Kurdish legislator Mahmoud Othman, though he would not say if Khalilzad was actually attending the meetings.

U.S. officials could not immediately be reached for comment, but have confirmed in recent weeks that Khalilzad was involved in discussions over last-minute &#34tweaks&#34 to the charter.

The United States is eager to see the passage of the constitution, since its rejection would prolong Iraq”s political instability for months — and could hamper the U.S. military”s plans to start pulling out some troops next year.

But there appeared to be too wide a gulf to get Sunni leaders to drop their opposition. While Shiite and Kurdish parties were willing to make some cosmetic additions to the draft, they rejected what they called central changes sought by Sunnis, particularly ones aimed at reducing the strong powers the charter gives to regional administrations over the central government.

&#34In general, there is no problem with making additions because it doesn”t contradict the principles of the constitution. But the amendments the Sunnis are demanding … are basic changes in these issues that absolutely won”t be accepted,&#34 Sheik Jalaleddin al-Saghir, an official in the Shiite-led United Iraqi Alliance, which dominates the government, told The Associated Press.

The Sunnis want changes to articles outlining the purging of members of Saddam Hussein”s former Baath Party — most of whose major figures were Sunnis — and others allowing provinces to join together into &#34regions&#34 under a single administration that would have considerable powers.

&#34We don”t want a federal system. It shouldn”t be a system of regions, it”s a system of provinces,&#34 Saleh al-Mutlaq, a prominent Sunni politician, said. He said the Sunnis want the articles on de-Baathification rewritten to &#34not single out the Baath Party.&#34

Shiite and Kurdish parties staunchly support the federalism provisions. Many of the same issues brought up by Sunnis were the subject of rancorous debate during the drafting of the constitution, which ended with the Shiites and Kurds approving the draft for the referendum over Sunni opposition.