A car bomb tore through a crowded market in northwestern Pakistan on Wednesday, killing 57 people hours after U.S. Secretary of State Hillary Rodham Clinton arrived in the country to show American support for its campaign against Islamist militants.

More than 100 people were wounded in the blast in the main northwestern city of Peshawar, the latest in a surge of bloody attacks this month by suspected militants apparently aimed at denting public backing for an army offensive against al-Qaida and Taliban close to the Afghan border.

The blast set scores of shops on fire and sent a cloud of gray smoke over the city. TV footage showed wounded people sitting amid the debris as people grabbed at the wreckage, trying to pull out survivors. One two-story building collapsed as firefighters doused it with water.

Clinton, on her first visit to Pakistan as secretary of state, was three hours' drive away in the capital of Islamabad when the blast took place. Speaking to reporters on her plane, she praised the army's new anti-Taliban offensive in South Waziristan and promised a new era in relations between Pakistan and the United States.

The explosion was in a neighborhood home to many Shiite Muslims, who have often been targeted by Taliban and al-Qaida allied Sunni extremists. It hit a market reserved for families. Many of the dead were believed to be women.

Emergency room doctor Zafar Iqbal said 57 people were killed and more than 100 were wounded in what police said was a suspected car bombing.

No group immediately claimed responsibility, but Peshawar has been the target of three of 10 major bombings or raids blamed on Islamist militants this month.

The attacks have killed more than 250 people. Most have targeted security forces, but at least four bombs have gone off in public places, apparently to sow fear and undercut support for the government.

The Taliban have warned Pakistan that they would stage more attacks if the army does not end an offensive in South Waziristan tribal region, where the military has dispatched some 30,000 troops to flush out insurgents.