A bombing at a bus station in a Shiite neighborhood in southwest Baghdad on Thursday killed at least seven people and wounded 31 others, police said, the latest in a series of deadly attacks ahead of a U.S. military withdrawal from cities next week.

Another three bombs and a mortar killed two more people around the capital. The U.S. military said nine American soldiers were wounded in two roadside bomb attacks against a convoy in eastern Baghdad. The attacks were latest is a series of deadly bombings mostly targeting Shiites in the past week.

The bombing occurred when a parked car bomb exploded inside the Baiyaa district's buses station, police officials said. They could not be named because they were not authorized to speak to the media.

The bombings came as police sifted through the bloody debris of an explosion late Wednesday that killed 78 people, trying to determine how such a massive bomb was smuggled into the teeming heart of Baghdad's Shiite Sadr City.

U.S. and Iraqi officials have warned that more bombs and attacks are to be expected in the days before and after U.S. troops complete a withdrawal from cities and major urban center on June 30. Most of the attacks so far have targeted Shiites or communities with predominantly Shiite populations. The killing spree began on June 20 with a massive truck bomb that killed 82 people in a mainly Shiite town near the northern city of Kirkuk, which was the deadliest bombing so far this year.

More than 160 people have died in bombings over the past five days.

In a statement, U.S. Ambassador Christopher Hill and top military commander Gen. Ray Odierno condemned the Sadr City bombing and said that "we deplore the senseless deaths and injuries of innocent Iraqi citizens."

Besides killing 78, the attack Wednesday in Sadr City also wounded 143 people. It was the deadliest in more than two years in the area, which is heavily controlled and where people entering the district have to pass through numerous checkpoints manned by Iraqi army and police.

According to Iraqi Army Maj. Gen. Qassim Atta, the bomb was built using about 200 kilograms (441 pounds) of high explosives packed with steel bearing and other metal objects. It was apparently loaded on a motorcycle pulling a cart.

"Most of victims of the explosion that occurred in Sadr city had small steel balls and nails in their bodies," said Dr. Mahmoud Mizaal at Sadr City hospital.

The increase in violence has raised concerns about the ability of Iraqi forces to protect their people after the withdrawal of U.S. forces, part of a security agreement that will see all American troops out of Iraq by 2011.

The explosion in Sadr City came only a few days after the U.S. military handed over to Iraqis its main base on the edge of the former Shiite militia stronghold. The district was the scene of fierce fighting between U.S. troops and Shiite militants last year.