The punishing strikes were carried out above approaching troops of the 2nd Brigade Combat Team, 3rd Infantry Division, which has battled insurgents south of the capital for months. The massive attack was part of Operation Phantom Phoenix, a nationwide campaign launched Tuesday against al-Qaida in Iraq.

A military statement said two B-1 bombers and four F-16 fighters dropped the bombs on Arab Jabour in 10 strikes on 40 targets. Al-Qaida fighters are believed to control Arab Jabour, a Sunni district lined with citrus groves and scarred by daily violence.

"Thirty-eight bombs were dropped within the first 10 minutes, with a total tonnage of 40,000 pounds," the statement said.

The attack came a day after the U.S. military reported that nine American soldiers were killed north of the capital in the first two days of a new offensive.

Many militants have fled U.S. and Iraqi forces massing north of Baghdad in Diyala province. Like Arab Jabour, Diyala is an agricultural area of palm and citrus groves that has defied the trend toward lower violence.

The campaign's scope is nationwide but is mainly focused on gaining control of Diyala and its most important city, Baqouba, which al-Qaida has declared the capital of its self-styled Islamic caliphate.

Six soldiers were killed and four were wounded Wednesday in a booby-trapped house in Diyala, the U.S. command said. It also announced that three U.S. soldiers were killed and two wounded in an attack Tuesday in Salahuddin province, north of Diyala.

The toll marked some of the deadliest days for U.S. forces in Iraq since last fall. For all December, 23 U.S. soldiers died in Iraq.

The blows against U.S. troops came as extremists tried to stay ahead of the military advance. Al-Qaida fighters retreated north from Diyala, presumably to Salahuddin, the top U.S. commander in northern Iraq, Maj. Gen. Mark P. Hertling, told reporters in Baghdad on Wednesday.

"Operational security in Iraq is a problem," he said, noting that the Iraqi army uses unsecured cell phones and radios. "I'm sure there is active leaking of communication."

Hertling said his troops had killed 20 to 30 insurgents in the first two days of the operation. It was unknown how many were killed in Thursday's strike.

Only Baghdad province has been deadlier than Diyala the past two years, according to an Associated Press count.

And while violence has declined over the past six months in Baghdad and many other places in Iraq, much of Diyala has remained a killing field. At least 273 civilians were slain in Diyala last month, compared to 213 in June. Over the same span, monthly civilian deaths in Baghdad dropped from 838 to 182.

The reason for the surge of bloodshed is that insurgents who were pushed out of the western province of Anbar and out of Baghdad shifted their operations into Diyala, U.S. commanders say.

The tree-lined farm region is more difficult terrain for fighting insurgents than the desert of Anbar, suggesting Diyala may not have seen the last of al-Qaida in Iraq. Compounding the difficulty for the military is the checkerboard pattern of Shiite and Sunni communities adjacent to one another.

The military will need a period of peace and stability to meet its goal of speeding up work on basic services and other civic projects that commanders believe will win more allies for the American effort.

In central Baghdad early Thursday, two bombs exploded nearly simultaneously close to a military checkpoint, killing two policemen and one soldier, police said. Eleven others were wounded in the attack, including four civilians.

In Kirkuk, 180 miles north of Baghdad, another pair of nearly simultaneous car bombings damaged two Christian churches and wounded two people Wednesday, according to a senior police officer there. Brig. Sarhat Qadir said the bombings at the Chaldean church of the Heart of Jesus and the Assyrian church of Mar Afram took place within 10 minutes of each other.

The churches, about 700 yards apart, were empty at the time of the attacks, which followed a series of bombings Sunday targeting three churches in Mosul, 225 miles northwest of Baghdad. No one was hurt in those attacks.

Prime Minister Nouri al-Maliki, a Shiite, told the Vatican's ambassador to Iraq on Tuesday that his government was working to ensure the safety of Christians.

Christians, who make up about 3 percent of Iraq's estimated 26 million people, have been frequent targets of attacks by militants since the 2003 U.S.-led invasion. Many have fled to neighboring countries.