A bomb blast Wednesday at Baghdad's largest and oldest wholesale market district killed at least 24 people and injured 35, according to the police.

The blast occurred at just before 10 a.m. at the Shurja commercial center, police Lts. Mohammed Khayoun and Bilal Ali Majid said. Shurja is one of Iraq's largest markets, where wholesalers sell food, clothing and house products to businessmen and shoppers.

Also Wednesday, an explosives-rigged bicycle detonated near an army recruiting center in southern Iraq, killing at least 12 people and wounding 28, police said.

Elsewhere, an oil pipeline exploded in southern Iraq on Tuesday, sparking a massive fire and killing at least 36 people and injuring 45, the Interior Ministry said. Police also found more than two dozen bodies across Baghdad as violence surged in the capital despite promising signs that a U.S. crackdown is curbing sectarian killings.

The U.S. military said three American soldiers and one Marine were killed the day before — two in combat in Anbar province and two from non-hostile causes. A fourth soldier died Tuesday in Baghdad. At least 13 American service members have died in Iraq since Sunday, the military said.

A man posing as a potential army recruit planted the bicycle bomb early Wednesday outside a recruiting center in downtown Hillah, about 60 miles south of Baghdad, said police Lt. Osama Ahmed. The bomb exploded as volunteers gathered to sign up for the army, Ahmed said.

The reason for the pipeline explosion was not clear, but police Lt. Raid Jabir said several people had been siphoning fuel when the blast occurred. Iraqis have faced severe fuel shortages since Saddam Hussein's 2003 ouster. Insurgents also frequent target pipelines and oil refineries.

The explosion occurred six miles south of Diwaniyah, the scene of fierce clashes between the Iraqi army and Shiite militia on Monday that left 73 people dead.

The latest violence came as U.S. and Iraqi officials claimed that a new operation in the capital has lowered Sunni-Shiite killings there, which had risen in June and July. On Monday, U.S. military spokesman Maj. Gen. William B. Caldwell said the murder rate in Baghdad had fallen by 46 percent from July to August and "we are actually seeing progress out there."

That figure could not be independently confirmed. But an employee of the main Baghdad city morgue, Muyaid Matrood, said that as of Monday, his office had received 337 bodies of people who had died violently this month, excluding bombing victims.

Health Ministry officials said about 1,500 violent deaths were reported in June as well as in July. Those figures included bombing victims. Even so, Deputy Health Minister Dr. Sabah al-Husseini said the previous surge in deaths had "obviously" diminished.

"In August, thank God, the number of deaths has evidently decreased," he said. "It's quite noticeable. Thank God, terrorist operations have decreased in Baghdad."

U.S. officials attributed the fall in sectarian killings to a major security crackdown launched Aug. 7. About 8,000 U.S. troops and 3,000 Iraqi soldiers were sent to the capital to search homes systematically and patrol the streets.

Similar operations in Baghdad and elsewhere have curbed violence for limited periods of time in the past, only to have killings flare again once American forces left the area.

It remained unclear if that would happen in Baghdad, but sharp violence did break out.

A total of 27 bodies were found Tuesday in three separate locations in Baghdad, police said. They included 11 bullet-riddled corpses discovered near a school in a Shiite neighborhood of south Baghdad and 13 more dumped behind a Shiite mosque in the west of the city.

Three others were found in the upscale Mansour neighborhood. In addition, four beheaded corpses, one believed to be an Iraqi soldier, were recovered from the Tigris River about 25 miles south of Baghdad, local officials reported.

Also Tuesday, seven people were killed in three separate mortar attacks in Baghdad, police said. A roadside bomb killed another person in western Baghdad, and one man died in a mortar attack in Mahmoudiya south of the capital, police reported.

The killings in Baghdad occurred one day after some of the sharpest fighting in months between the Iraqi army and Shiite militiamen loyal to radical cleric Muqtada al-Sadr. The clashes in Diwaniyah, 50 miles south of the capital, raged for nearly 12 hours.

On Tuesday, Prime Minister Nouri al-Maliki's office announced that 73 people had been killed in the Diwaniyah fighting, including 50 militiamen and 23 Iraqi soldiers. The toll was significantly higher than the 40 fatalities reported by local officials Monday.

Tribal leaders held reconciliation talks Tuesday to prevent retaliatory attacks and "life is back to normal," Jabir, the local police lieutenant, said.

"Shops are open and Iraqi police and soldiers are deployed everywhere in Diwaniyah," Jabir said by telephone.

He said militiamen had withdrawn from all the areas they had seized. Coalition helicopters were flying over the area on Tuesday, said Sheik Abdul-Razaq al-Nidawi, al-Sadr's representative in Diwaniyah.

The Diwaniyah fighting was significant because it pitted mostly Shiite Iraqi soldiers against the militia of one of the country's most prominent Shiite leaders. It also illustrates the complexity of the security crisis in Iraq — with Sunni insurgents fighting U.S. troops in the west, Sunnis and Shiites killing one another in Baghdad and now Shiites battling Shiites in the south.

Al-Sadr led two uprisings against U.S. forces in 2004 but has since emerged as a major political figure, controlling 30 of the 275 seats in parliament and five Cabinet posts.

In Baghdad, Abdul-Aziz al-Hakim, head of Iraq's biggest Shiite party and a rival of al-Sadr, described the fighting as "annoying and painful."