Iraq's seemingly inexhaustible corps of terror bombers struck across the country again Thursday, from a restaurant in Baghdad's teeming Sadr City, to a police station leveled by a blast near the Syrian border. At least 15 people were reported killed.

The new blows against Iraqi government authority came as U.S. military casualties rose toward a four-year death toll of 3,500, with 22 dead reported for the first six days of June, almost double that of June 2006.

The U.S. military reported four U.S. soldiers were killed in separate incidents Tuesday and Wednesday — roadside bombings in eastern Baghdad and near Beiji, north of the capital, and an explosion and enemy gunfire in Diyala province, northeast of Baghdad.

That lifted the U.S. death toll in four years of war to 3,498. And it raised the average rate of U.S. troops deaths to about four per day in June, compared with two a day in June 2006.

The Bush administration has warned that the current troop buildup in and around Baghdad will result in more U.S. casualties as American troops increasingly come into contact with enemy forces.

Brig. Gen. Kevin J. Bergner told reporters Wednesday that the last of five brigades earmarked for the buildup will arrive in the "next couple of weeks," but may take up to two months to establish itself as fully operational.

In the capital's eastern Sadr City district, a Shiite Muslim stronghold, a bomb beneath a parked car exploded at lunchtime outside a falafel restaurant, police reported. At least three people were killed and eight wounded, said a police officer, speaking on condition of anonymity because he wasn't authorized to release the information.

Sadr City has been repeatedly targeted by Sunni extremists seeking to terrorize Iraq's Shiite majority and inflame hostilities between the Muslim sects.

Earlier, in the day's first reported attack, a homicide bomber blew up his explosives-laden truck at about 9 a.m. at a police station in Rabia, near Iraq's border with Syria, killing at least four policemen and five civilians, and wounding 22 other people, an Iraqi army spokesman said.

A guard shot the driver as he approached the building, but the truck still penetrated its blast walls and exploded, destroying the one-story structure, said Capt. Mohammed Ahmed of the army's Third Division. Rabia is 50 miles west of Mosul and about three miles from Syria.

An hour later, in Ramadi, 70 miles west of Baghdad, three policemen were killed and four others wounded when a homicide driver blew up his automobile at their checkpoint, police said.

The post was just 50 yards from the traffic police headquarters, said another police officer.

In other attacks Thursday, mortar shells landing in two districts of western Baghdad killed two civilians and wounded 12 others, police reported.

On the offensive early Thursday, a joint Iraqi-American force raided locations in Sadr City and detained 16 suspected members of a "secret cell terrorist network" believed helping transport weapons, including advanced roadside bombs, from Iran to Iraq, the U.S. command reported.

An Iraqi police officer said two on-duty policemen in the area were wounded by random fire from the raiding party.

At midday, in another incident, an exploding roadside bomb targeting a U.S. patrol wounded two Iraqi civilians in the Neiriya area of eastern Baghdad, and two others were wounded when the Americans opened fire randomly at the scene, an Iraqi police officer said.

All the Iraqi police officers spoke on condition of anonymity because they were not authorized to release the information and feared repercussions.

The U.S. command press information center said it had no immediate information on the casualties in those two incidents.

In another development, the British ambassador to Iraq, Dominic Asquith, issued an appeal to the kidnappers of five Britons, held since May 29, to release them or open negotiations.

The five — four security guards and a consultant — were abducted from the Iraqi Finance Ministry by some 40 heavily armed men who then rode off with them in the direction of Sadr City.

Iraqi officials have said they believe they were taken by the radical Shiite Mahdi Army militia, possibly in retaliation for the killing by British forces of the militia's commander in the southern city of Basra.

"I ask those holding them to release them so they may return to their families," Asquith said. Then, in a clear offer to consider demands, he added, "We have people here in Iraq who are ready to listen to any person about this incident, or any person who may be holding these men and who may wish to communicate."