Baghdad, Iraq ? Sectarian killings have surged in parts of Baghdad not yet included in a security offensive, the U.S. military said Thursday, while bombings and other insurgent attacks killed four American soldiers and wounded 25 in the capital region.

Police reported finding 20 bodies dumped on streets, many of them victims of reprisal killings in the escalating conflict between Shiite and Sunni Arabs. Six people died when a car bomb exploded at a soccer field in Fallujah, raising the day’s death toll across Iraq to at least 28.

One of the few positive developments for the U.S.-led coalition and the national unity government was the reported killing of a senior member of al-Qaida in Iraq and the capture of another.

Shiite politicians, meanwhile, said they had made progress in trying to break a deadlock on legislation to establish autonomous regions as part of a federated Iraq. Sunni Arabs oppose the bill, fearing it could split Iraq into three sectarian and ethnic cantons.

The worst violence in Baghdad came at midday when a suicide car bomber attacked U.S. troops on the western outskirts of the city, killing two Americans and wounding 25, the U.S. command said. It said six of the wounded had returned to duty and 15 were not seriously injured.

An Iraqi man fans his relative, injured by a car bomb explosion, with a piece of cardboard in a hospital in Baghdad, Iraq. Car bombs and drive-by shootings Thursday killed at least 28 people and wounded dozens of others in a series of attacks across Iraq, officials said.

Two more U.S. soldiers also were killed in the Baghdad area, one when his patrol was hit by small-arms fire and the other when a roadside bomb exploded.

The command also announced the killing of a soldier Wednesday near the northern city of Mosul.

Violence has intensified during the past two days, with more than 140 people either killed by attacks or their bodies found dumped in the streets of Baghdad.

“There was a spike in violence in Baghdad over the past 24 hours from murder-executions,” said Maj. Gen. William B. Caldwell, the U.S. command’s spokesman. “Most of those are associated with sectarian violence, not all necessarily, but a large portion.”

Iraq’s Interior Ministry reported that Abu Jaafar al-Liby, who it described as either the second or third most important figure in al-Qaida in Iraq, was killed by police earlier this week.