Thousands of Iraqi troops launched a crackdown in Kirkuk, ordering residents to stay in their homes in an effort to put down violence that has swelled in the north amid efforts to rein in bloodshed in Baghdad.

Elsewhere in the north on Saturday, a homicide bomber rammed a police checkpoint with an explosives-packed car, killing 14 people in the town of Tal Afar. It was the deadliest attack in a day that saw 26 Iraqis killed around the country.

In March, U.S. President George W. Bush had pointed to Tal Afar as an example of progress made in bringing security to Iraq after a major U.S. offensive swept through the town, 30 miles (50 kilometers) from the Syrian border.

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But after a period of quiet, Saturday's was the fourth homicide attack in the town in the past three weeks.

Thousands of U.S. and Iraqi troops have been carrying out an intensified sweep of Baghdad since August, searching neighborhood by neighborhood to root out insurgents and militias who have killed thousands this year.

But at the same time, shootings, bombings and other attacks has been swelling in northern areas such as Kirkuk and Mosul, the capital of the province where Tal Afar is located — though the bloodshed has not been on the scale seen in Baghdad.

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The U.S. military announced Saturday that a U.S. soldier was killed Friday near Beiji, about 60 miles (100 kilometers) southwest of Kirkuk.

Authorities in Kirkuk this week completed digging a 10-mile (16-kilometer) trench around the city's southern and western edges aimed at cutting off side roads to prevent car bombs from being brought into the city. Intensified checkpoints were set up on city entrances that remained.

On Saturday, authorities announced a curfew had been extended to round-the-clock "until further notice," ordering all residents off the streets, said Kirkuk police chief Lt. Gen. Sherko Shaker.

"This operation comes as a measure to cleansing Kirkuk from weapons, as well to prevent the militants from having any chance to reorganize their abilities," he said. "We shouldn't give them any chance to rest."

Troops conducted searches and raids on Saturday in the southern and western sectors of the city, where most of the Sunni Arab population is centered. So far, 150 suspected Sunni insurgents have been arrested and more than 220 assault rifles have been seized, Shaker said.

Kirkuk, located 180 miles (290 kilometers) north of Baghdad, is a major oil center and the focus of an ongoing struggle for power between its large Sunni and Kurdish populations.

The Kurds want to include the city in their autonomous zone further north and are working to resettle thousands of Kurds who were driven out during the regime of Saddam Hussein and replaced with Sunni Arabs.

Al Qaeda in Iraq and another major Sunni group, Ansar al-Sunna, have increased their presence in regions west of the city, said Sheik Abdul-Rahman al-Munshid, a top sheik in the Sunni Obeid tribe. He blamed Kurdish efforts in the city for fueling Sunni Arab support for insurgents.

"The demands of the Kurdish political forces and their attempt ... to work to make Kirkuk part of the northern region that have created worry among the non-Kurdish groups," he said.

Meanwhile, a suicide bomber rammed an explosives-packed vehicle into a police checkpoint in Tal Afar, killing four policemen and 10 civilians. Some of them died when parts of nearby homes collapsed from the force of the blast.

In nearby Mosul, gunmen killed a woman whose son works with the city police, Mosul police Col. Abdel-Karim al-Jubouri said. A Kurdish lawyer was shot to death in front of his home in the evening.

Gunmen killed five people in separate shooting attacks on roads northwest of Mosul, including three Iraqis who worked at a nearby military base.

In Baghdad on Saturday, gunmen sprayed a Shiite-owned bakery in the Mansour district with bullets, killing two people and wounding one.

More bodies with bound hands and signs of torture on them were found; two were in the Tigris River in downtown Baghdad and five others were dumped in a southeastern suburb.

Meanwhile, the U.S. command said it had captured 28 suspected terrorists in a series of nine raids early Tuesday in the Jisr Diyala neighborhood of southeastern Baghdad. Among those were three "high value individuals, including the No. 9 person on the division's high-value target list."

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