BAGHDAD — Iraqi police on Saturday closely guarded two bridges in Baghdad that were heavily damaged by suicide car bombers in what appears to be a new strategy by suspected Sunni insurgents of targeting crossovers in the capital.

Friday’s attacks in predominantly Shiite areas of the city brought to five the number of bridges that have been targeted by large explosions in Baghdad since March 21.

It remains unclear whether the main goal of Iraqi insurgents is to spark sectarian violence by targeting bridges that unite predominantly Shiite and Sunni areas of the city, or to knock out vital supply and transportation links.

In another development, The New York Times reported Saturday that a draft of a new American government report says that between 100,000 and 300,000 barrels a day of Iraq’s declared oil production over the past four years is unaccounted for and could have been siphoned off through corruption or smuggling. Using an average of $50 a barrel, the report says the discrepancy was valued at $5 million to $15 million daily, the paper said.

The draft report comes as the U.S. and Iraqi governments are under pressure to show progress in Iraq by raising oil production levels, which have been well below the U.S. goal of 3 million barrels a day. Virtually the entire economy of oil-rich Iraq is dependent on oil revenues.

Friday’s suicide car bombers attacked two bridges that cross the Diyala River, a tributary of the Tigris, and are located about 2.5 miles apart in southeastern Baghdad.

The two attacks occurred within moments of each other at about 6 p.m. on Friday, the traditional Islamic day of worship, and killed at least 23 Iraqis and wounded 57, including police at checkpoints and civilians driving or walking across the bridges, police said.

In all, at least 52 Iraqis were killed or found dead Friday in sectarian violence in Iraq. Also Friday, the military announced that two U.S. soldiers were killed in separate bombing attacks the day before — one in Diyala province north of the capital and the other in Baghdad.

Dozens of people, meanwhile, took to the streets in the Diyala provincial capital of Baqouba to demand the release of four women they said were detained Friday by U.S. troops. The U.S. military had no immediate comment on the protest.

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

On Saturday, the old Diyala bridge, which American forces had rebuilt after destroying it at the start of the Iraq war, had one of its two lanes open to traffic and pedestrians. Police kept everyone away from a large hole blown through the concrete span over the Diyala River.