By Mustafa Mahmoud

KIRKUK, Iraq, Oct 7 (Reuters) - Thousands of Iraqi police and soldiers launched a major security crackdown in the restive Iraqi oil city of Kirkuk on Saturday, searching homes for weapons after all residents were ordered off the streets.

In northern Tal Afar, northwest of Kirkuk, a suicide car bomber killed 14 people in an attack on an Iraqi army checkpoint, the latest in a series of deadly suicide bombings in the town since the start of the Muslim holy month of Ramadan.

The fresh bloodshed followed a warning by U.S. Senate Armed Services Committee Chairman John Warner who said Iraqi Prime Minister Nuri al-Maliki's government had just 60 to 90 days to control the violence that threatens civil war or the United States would have to reconsider its options.

Kirkuk, 250 km (155 miles) north of Baghdad, is an ethnically mixed city claimed by Arabs, Kurds and Turkmen which has seen an upsurge of violence. A spate of near simultaneous car bombs in the city killed 20 people on Sept. 17 alone.

Kirkuk police chief Major General Shirko Shakir said cars and pedestrians had been cleared from the city's streets after an indefinite curfew was imposed on Friday night and Iraqi security forces began sweeping through neighbourhoods.

"This operation is an attempt to control the deterioration of the security situation in the city. We will continue it until we clean up the city and end insurgent activity," he said.

Iraqi police Major General Jamal Taher said a 15 km trench had been dug south of the city in the last week to try to prevent insurgents and car bombs from entering the city.

Iraqi forces have beefed up security in many cities, fearing an increase in violence with the start of Ramadan.

Saturday's car bomb attack in Tal Afar was the fourth suicide car bombing on an army or police checkpoint in the town since the start of the holy month two weeks ago.

The town has been largely free of violence since U.S.-led forces drove out al Qaeda militants in a 2005 offensive. In March, U.S. President George W. Bush cited Tal Afar as an example of progress being made in Iraq.

LAWMAKER'S DEATH CONDEMNED

Tal Afar residents said the surge in violence came after the Iraqi military and police dramatically increased the number of checkpoints in the town, setting up many in residential areas.

"We are living in a state of terror," one resident told Reuters. "My house is near an army checkpoint, which is a target for suicide bombers. If they attack it, the soldiers will be killed and so will we."

Police Colonel Kareem Khalaf said 10 civilians were among the 14 killed in Saturday's attack.

Maliki's four-month-old national unity government has failed to control the spiralling violence that has killed thousands of Iraqis, despite a series of plans aimed at ending sectarian and insurgent bloodshed and reconciling Shi'ites, Sunnis and Kurds.

Gunmen this week abducted and killed a Kurdish lawmaker in the capital, where U.S. and Iraqi forces have launched a major operation to regain control of the city's streets. He was the first member of the parliament sworn in in March to be killed.

The U.S. envoy to Iraq and the top U.S. military commander General George Casey condemned the killing in a statement on Saturday as a desperate attempt to derail Iraq's progress towards "freedom and prosperity".

U.S. Secretary of State Condoleezza Rice paid a surprise visit to Baghdad this week to deliver a blunt message to Iraq's leaders, telling them to end their "political inaction" and work faster to end the violence threatening to tear Iraq apart.

U.S. officials say sectarian conflict between majority Shi'ites and minority Sunni Arabs has overtaken the insurgency against the Shi'ite-led government and U.S. forces as the main cause of the attacks that kill some 100 Iraqis each day.

But the insurgency is continuing to exact a deadly toll on U.S. soldiers. The U.S. military said a soldier had been killed in action near the northern oil refinery town of Baiji on Friday, bringing to at least 24 the number of U.S. soldiers killed in the last week.

U.S. military spokesman Major General William Caldwell said on Wednesday it had been a "hard week" for U.S. soldiers, who suffer two to three deaths a day on average in Iraq. (Additional reporting by Thaer Ismail in Mosul, Sherki Raouf in Sulaimaniya Ahmed Rasheed and Mussab Al-Khairalla in Baghdad, and Donna Smith in Washington)

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