Twin homicide car bombers struck police checkpoints at bridges in a predominantly Shiite area of Baghdad on Friday, killing at least 23 people just hours after a series of U.S. raids on car bomb networks around the capital killed four suspected insurgents.

The blasts underscored the difficulty U.S.-led forces were having in destroying the presumably Sunni insurgent cells that have stepped up car bombings in the capital since the U.S. began its security crackdown 12 weeks ago.

"They have actually pushed back," U.S. spokesman Lt. Col. Christopher Garver said, referring to the insurgents. "We've been saying this all along, the enemy is determined."

In a sign of that determination, the top U.S. commander in northern Iraq said he doesn't have enough troops to crush Al Qaeda-led insurgents in Diyala, a province northeast of Baghdad and scene of recent sharp fighting.

Maj. Gen. Benjamin R. Mixon had already received extra troops for Diyala and has increased attacks on militants. But he told Pentagon reporters in a video conference that he has asked Lt. Gen. Raymond Odierno, the No. 2 commander in Iraq, for more.

"I laid out a plan for Gen. Odierno on the numbers of forces that I would need," Mixon said. "We have made progress ... we have taken terrain back from the enemy. Gen. Odierno intends to give me additional forces as they become available."

Also Friday, the military announced that two U.S. soldiers were killed in separate bombings the day before. One soldier died in Diyala and the other in Baghdad, the military said.

In all, at least 52 Iraqis were killed or found dead Friday in politically related violence.

The suicide car bombers struck about 6 p.m. The driver of a sedan waiting in a line of cars at a police checkpoint near the old Diyala Bridge blew up his vehicle, partially collapsing the span, police said.

About two minutes later the driver of a large fuel truck barreled toward a second checkpoint at the nearby new Diyala Bridge and set off his explosives, police said. The bridge was also damaged, and firefighters struggled to extinguish burning police and civilian cars that had been driving across during the attack.

The blasts sent smoke billowing over the Shiite Zafaraniyah area of southeastern Baghdad.

"Suddenly I heard a big explosion, and a huge fire rose from the checkpoint," said Abdullah Khalaf, who was selling sheep by the side of the road when the fuel truck raced past. He ran with his son to the checkpoint and saw the truck and three cars on fire, he said.

"I saw one wounded woman asking for help while she was trapped inside a damaged car and three wounded policemen on the ground," he said. "There were pools of blood and pieces of flesh."

The bombings at the bridges, which cross the Diyala River, a Tigris tributary, killed 23 people, including 11 police officers, and wounded 57 others, 26 of them police, police said.

Baghdad's bridges repeatedly have been targeted by bombers. The most serious had been the April 12 truck bombing that collapsed the steel-girder Sarafiyah bridge, plunging cars into the water and killing 11 people. Two days later, a suicide car bombing killed 10 people at the Jadriyah bridge.

Officials say Al Qaeda-linked Sunni insurgents are using the bombings to try to provoke retaliatory violence from mainly Shiite militias that had agreed to lay low to avoid confrontations with Americans during the security crackdown.

The blasts Friday came despite a series of measures aimed at reducing violence in the capital.

U.S. and Iraqi forces have increased checkpoints and, in a bid to prevent bridge attacks, banned trucks capable of carrying more than one ton from crossing without strict searches. They also have long imposed a four-hour weekly driving ban during Friday prayers in Baghdad, but that ban ended on schedule three hours before the attacks.

U.S. forces have also tried to break up the insurgent cells behind the bombings.

"We have been focusing very heavily on the car bomb factories," Garver said. "We've been targeting the networks and the people that move the bombs and make the bombs as well."

In one raid early Friday, troops came under fire as they approached a building near Taji, an air base 12 miles north of Baghdad, suspected of housing a cell responsible for car bomb attacks on civilians and security forces, the military said.

The troops returned fire, killing four armed men, including one suspected of being a cell leader with ties to Al Qaeda in Iraq's chiefs, the military said.

Forces also carried out a series of raids in Baghdad and the northern city of Mosul on Thursday and Friday, detaining nine people suspected of producing bombs and smuggling foreign fighters into the country to carry out attacks against U.S. troops, the military said.

In Washington, David Satterfield, Secretary of State Condoleezza Rice's top adviser for Iraq, said the United States knows for certain that radical Shiite cleric Muqtada al-Sadr is living next door in Iran.

"We know he's out of the country, we don't (merely) think" so, Satterfield told The Associated Press, disputing aides to the anti-American religious and political leader. "He's in Iran, which is where he has been since mid-January."