Two major armed factions opposed to Iraq's still unfinished constitution on Sunday ordered their followers to register for an upcoming vote on the new charter, with one faction urging a vote against it and the other telling supporters to wait for further orders. "Rejecting the constitution will defeat the American plan in Iraq," Ansar al-Sunna, the most prominent Sunni insurgent movement to join the call for voter registration, said in a statement. Voting amounts to "jihad against the Americans," it said.

The rebellious Shiite cleric Moqtada Sadr, who leads the other faction, told his followers to register ahead of the Sept. 1 deadline but to "wait for our order to come later on whether to vote or not." Sadr issued his statement after residents of the Shiite holy city of Najaf asked what they should do.

Many Sunnis, including moderates, oppose provisions in the draft constitution that could allow a strong Shiite federal state in the southern part of the country. A mobilization of Sadr's followers against the document could lead to its defeat. If two-thirds of voters in three of Iraq's 18 provinces reject the charter, the constitution will fail.

Last week, Iraqi political leaders, unable to resolve several sticking points, pushed back their original Aug. 15 deadline for completing the constitution by one week. On Sunday, Shiite and Kurdish leaders met in Baghdad's concrete-walled Green Zone to work out remaining differences over several issues, including oil revenues, ahead of their new deadline on Monday.

Shiite, Kurdish and Sunni leaders planned a last round of talks Monday morning. Laith Kubba, a government spokesman, suggested that the deadline might be extended by yet another week if the push failed.

Iraqis are scheduled to vote on the constitution on Oct. 15, and elect a new, full-term government in December. Political leaders are under intense U.S. pressure to stick to that timeline, which would allow a significant withdrawal of U.S. troops by spring.

Sadr, whose forces battled U.S. troops in 2004, previously has rejected taking a personal role in the government-building as long as U.S. troops are on Iraqi soil. He refused to vote in January in national elections that seated the interim government, but told his followers to decide for themselves whether to take part.

Sadr also opposed a federal system backed by Kurds and some Shiites, saying that regional autonomy would lead to the breakup of Iraq. In a statement issued earlier this summer, Sadr appeared to relax his position, saying that the issue should not be settled while U.S. forces were still in Iraq.

Debate on the constitution, and in particular on provisions for a Shiite federal state in the south, have opened fault lines both between the two main Shiite blocs in government and between Abu Musab Zarqawi, a Jordanian insurgent leader, and his presumed Sunni allies.

Many in the Sunni minority heeded warnings from their leaders and insurgents to boycott the January vote. As a result, the once-dominant group saw its political clout diminish. Many Sunni leaders have made clear that they plan to vote next time around.

"We are all eager to participate in the referendum -- first, to prove we are Iraqis, and second, to fail the federalism project the Shiites demand," said Safa Hiti, a professor of criminal law at Anbar University in Ramadi. The Sunni insurgent faction issuing the call Sunday is made up of seven groups. Six of them signed a joint statement backing the referendum. "Because voting is a jihad of word, and doesn't differ from the jihad by sword, there is no objection to participating in the referendum to show the world your numbers and strength and to defeat federalism," the statement said. The groups promised they would not attack U.S. forces on the day of the referendum, so as "to protect people who go to vote."

Ansar al-Sunna issued a separate statement.

"I think our voice goes with all those urging Iraqis to participate, whether they say 'yes' or 'no,' " said Kubba, the government spokesman, noting the call by some "violent groups" for Iraqis to vote against the constitution.

Speaking at a news conference, Kubba also accused relatives of former president Saddam Hussein of working from neighboring Jordan to destabilize Iraq and revitalize the banned Baath Party. Kubba did not specify which family members, but Hussein's two oldest daughters live in the Jordanian capital, Amman. Kubba's remarks were the strongest yet directed against Jordan, a U.S. ally. Tens of thousands of Iraqis have moved to Amman to escape the violence in Iraq.

Bloodshed continued in the country Sunday. Suspected insurgents killed a translator working with the U.S. military in Tikrit and three members of his family, said Capt. Nazhan Saad Foad of the Iraqi security forces. Witnesses said the victims had been handcuffed and beheaded.

A car bomb in northern Baghdad exploded at a restaurant on the Tigris River, killing four Iraqis, police Lt. Ahmed Murtadha said.

A roadside bomb killed an American soldier near Tikrit, about 90 miles north of Baghdad, the U.S. military said.

Separately, the U.S. military said it had ordered a criminal investigation into the death of Mohammed Sumaidaie, 21, a cousin of Iraq's ambassador to the United Nations. The ambassador has alleged that U.S. Marines killed his unarmed relative in June during a raid in western Iraq.

Special correspondent Saad Sarhan in Najaf contributed to this report.

Shiite cleric Moqtada Sadr has said the issue of federalism should be decided after U.S. forces leave Iraq.