TAL AFAR, Iraq (AP) – A joint U.S.-Iraqi force punched deep into Tal Afar, a key insurgent staging ground near the Syrian border, and the Iraqi army said Thursday it arrested 200 suspected militants in the sweep – three-fourths of them foreign fighters.

Most of the estimated civilian population of 200,000 have now fled this predominantly Turkmen city, where 70 percent of that ethnic group is Sunni Muslim – the sect that dominates the Iraqi insurgency. The U.S. military reported killing seven insurgents over the past two days amid growing indications the joint force was preparing to intensify the operation.

The sweep in Tal Afar came as election officials tallied figures from three Sunni-dominated provinces, where the voter registration was extended a week in preparation for the Oct. 15 nationwide referendum on the new constitution.

“Turnout was unbelievable and people were very enthusiastic, especially in Fallujah and Ramadi,” said Farid Ayar, an electoral commission spokesman in Baghdad. Those cities are Sunni insurgent bastions in Anbar province, which stretches west from Baghdad to the Syrian, Jordanian and Saudi borders.

The large voter signup suggests minority Sunnis are mobilizing to defeat the draft charter, a marked tactical shift from January, when their boycott of the parliamentary election handed control of the 275-member National Assembly to Shiites and Kurds.

The new basic law was approved and sent to voters by a coalition of Shiites and Kurds, over the objections of Sunni representatives, who fear it would allow the country to split into sectarian and ethnic mini-states. That could cut Sunnis out of Iraq’s enormous oil wealth.

The very Sunni clerics who railed last January against an election “under foreign military occupation” are now urging their people to take part in both the referendum and the parliamentary balloting in December.

Rejection of the charter would mean elections in December for a new parliament under the rules of the interim constitution approved in March 2004. The new parliament would start the entire process of drafting a constitution from scratch.

Demographics are a big problem for the Sunni Arabs – an estimated 20 percent of Iraq’s 27 million people.

Sunnis form the majority in four of the 18 provinces, but their numbers are overwhelming in only two, Anbar and Salahuddin. Under election rules, a “no” vote by a two-thirds majority in any three provinces would defeat the referendum.

In Anbar and Salahuddin approximately 75 percent of eligible voters signed up by the Wednesday deadline, election officials said, while cautioning the tally was not final. The percentage figure changed throughout the day as more regions reported.

In Diyala, a Sunni majority province where the count was final, 417,000 of 750,000 eligible voters, or 56 percent, registered, according to Amir Latif, director of the provincial elections commission. He spoke from the provincial capital of Baqouba, 35 miles northeast of Baghdad.

Sunnis also hold a majority in Ninevah province, home to Mosul – Iraq’s third-largest city – and Tal Afar. But much of the Sunni population in the province is Kurdish and committed to the draft charter.

In the Tal Afar sweep, Iraqi army Capt. Mohammed Ahmed said one of the captured insurgents was Amr Omayer, an Iraqi who allegedly was the most-wanted militant in the city and the commander of all insurgent operations launched from there.

Ahmed said some of those arrested could not speak Arabic.

“We believe they are Afghans, but we have not checked their nationalities so far,” he said. The Arab-speaking captives were from Syria, Sudan, Yemen and Jordan, he said, adding that the approximately 50 Iraqis rounded up in the sweep carried fake identity papers.

The joint force has reported heavy fighting around the perimeter of the city for several days and deadly bombings that mainly have killed civilians. Iraqi authorities said 80 percent of the civilian population has fled the city, about 260 miles north of Baghdad and 35 miles from Syria.

“We ordered the families to evacuate the Sunni neighborhood of Sarai, which is believed the main stronghold of the insurgents,” Ahmed said, suggesting it soon would be targeted in a major push.

Eight civilians were killed in the city Wednesday by a suicide car bomber at an Iraqi checkpoint, he said.

The U.S. military is no stranger in Tal Afar – a haven for insurgents crossing into Iraq from Syria. After the ouster of Saddam Hussein, the United States installed a largely Shiite leadership in the city, including the mayor and much of the police force.

The Sunni majority complained of oppression by the government and turned to the insurgents – who are mainly Sunnis – for protection.

American forces swept through last fall, and the local police chief declared the city insurgent-free.

But after the operation, U.S. forces quickly scaled back, leaving behind only about 500 soldiers to coordinate with newly trained Iraqi forces. The joint force was unable to prevent insurgents from retaking entire neighborhoods.

In other developments:

• Police reported finding 17 bullet-ridden bodies near the capital. Fifteen were spotted close to Mahmoudiya, a heavily Sunni farming town about 20 miles south of Baghdad. Two more bodies, blindfolded and handcuffed, were found on Baghdad’s southern outskirts near a sewage treatment plant. The victims were not identified.

• The American military announced the death Wednesday of a soldier assigned to the 2nd Force Service Support Group in an industrial accident at Camp Taqaddum near Fallujah, 40 miles west of Baghdad. At least 1,895 members of the U.S. military have died since the beginning of the Iraq war in March 2003, according to an Associated Press count.