Bombs killed more than 60 people and wounded more than 200 Sunday in Baghdad and the northern oil center of Kirkuk — a dramatic escalation of violence as U.S. and Iraqi forces crack down on Iraq's most feared Shiite militia.

Prime Minister Nouri al-Maliki left Sunday for talks in Washington this week with President George W. Bush to discuss sectarian violence that has risen sharply since Iraq's national unity government took office two months ago.

The Baghdad blast occurred when a suicide driver detonated a minivan at the entrance to a bustling market in Sadr City, the capital's biggest Shiite district and stronghold of radical cleric Muqtada al-Sadr and his Mahdi Army militia.

CountryWatch: Iraq

At least 34 people were killed and 74 were wounded in the blast, the Iraqi army said in a statement. Eight more people died and about 20 were injured when a roadside bomb exploded two hours later at a municipal building in Sadr City about one half mile from the car bombing, the army said.

In Kirkuk, 180 miles to the north, a car bomb detonated at midday near a courthouse located amid a cluster of wooden shops and stalls, many of which burst into flames, engulfing the warren of crowded streets in roiling black smoke.

Twenty people were killed and 159 were injured, police said. The tally of injured was so high because many people were trampled as panic swept shoppers, police said. Others suffered burns when the initial blast triggered secondary explosions in shops that sold chemicals and flammable liquids, police said.

Scenes at local hospitals were gruesome. Victims young and old lay bleeding on stretchers and gurneys, some of them scarred with horrific burns. Many lay unattended as doctors and nurses scrambled to care for the large number of wounded.

It was the fourth car bombing this month in Kirkuk — the center of Iraq's vast northern oil fields. Tensions have been rising in Kirkuk because the area's Arabs, Kurds and Turkomen all have rival claims to the region.

The Sadr City car bombing was the second major suicide attack this month in the teeming slum district, where al-Sadr's Mahdi militiamen rule the streets. A July 1 suicide bombing in Sadr City was followed by a wave of reprisal killings of Sunnis.

Many Sunni politicians hold the Mahdi Army responsible for the wave of attacks against Sunnis that followed the Feb. 22 bombing at a Shiite shrine in Samarra. Al-Sadr's aides deny that the militia is doing any more than protecting Shiites from attacks by Sunni extremists including Al Qaeda in Iraq.

So feared is the militia among Sunnis that many of them refer to any band of armed, masked Shiites as the Mahdi Army.

Hours before the Sunday blast, Iraqi troops and U.S. advisers launched raids in Sadr City and the mostly Shiite district of Shula, searching for suspected members of sectarian death squads, a U.S. statement said.

Two hostages were freed in the Sadr City raid, and two people were arrested in Shula, U.S. and Iraqi officials said.

U.S. officials made no mention of al-Sadr or the Mahdi Army in statements about the raids.

"We are not concerned with whom they are affiliated. We are only concerned with taking people responsible for these illegal acts off the streets and will continue to do so aggressively," U.S. military spokesman Lt. Col. Barry Johnson said.

However, it was clear that the U.S.-led coalition is stepping up pressure against the Shiite militia in a bid to reduce sectarian violence, which U.S. officials now consider a greater threat than the Sunni-led insurgency.

Last week, British troops arrested the commander of Mahdi Army forces in Basra, Iraq's second largest city. On Saturday, U.S. and Iraqi troops killed 15 fighters in a three-hour gunbattle around al-Sadr's office in Musayyib. 40 miles south of Baghdad. One Iraqi soldier was killed, the U.S. said.

Local officials also said U.S. and Iraqi troops Saturday raided al-Sadr's office in Mahmoudiya, scene of reprisal killings since suspected Sunni gunmen killed 50 people, mostly Shiites, in a raid last week on a market.

In Najaf, a senior al-Sadr lieutenant claimed the Americans want to crush the radical cleric's movement because he is the most prominent Shiite leader to oppose the U.S. military presence.

"We are the only group that rejects the occupation because we are nationalists," said Jalil al-Nouri. "We are the only political group that rejects their presence in the country and we demand that they leave. We are to the point, and we are clear."

Al-Sadr, scion of one of Shiite Islam's most prominent clerical families, led two uprisings against the Americans in 2004. The U.S. stopped short of capturing him under pressure from the Shiite clerical hierarchy.

After a cease-fire, al-Sadr has emerged as a major political force. He controls a movement modeled after Hezbollah in Lebanon, running charities, clinics — and an armed militia. Al-Sadr's followers hold 30 of the 275 seats in parliament and control five Cabinet ministries including health, transportation and agriculture.

Although al-Maliki has pledged to disband the militias, he relies on al-Sadr for political support. The prime minister's Dawa party was founded by al-Sadr's father-in-law, who was executed under Saddam Hussein.

CountryWatch: Iraq