Thousands of US forces continued to search for three American soldiers feared captured by al-Qaeda last week after an attack on their convoy south of Baghdad also killed four US troops and an Iraqi soldier.

Meanwhile, clashes broke out in the mostly Shia city of Nasiriyah in southern Iraq on Wednesday, when a militia fought with police there after they arrested two wanted militia members, police said. Nine Iraqis were killed and 75 wounded, a police spokesman said.

A US government report released Tuesday showed that the recent US troop increase and security crackdown has had little effect on the high number of attacks in the country.

The average number of attacks rose from 71 a day in January 2006 to a high of 176 per day in October, according to the report from the Government Accountability Office. In February, when the troop increase began to take effect, daily attacks dropped slightly to 164. Daily attacks averaged 157 in March and 149 in April, the report said.

The report, which cited the US-led forces in Iraq for the figures, did not measure the numbers killed and wounded in the attacks.

Military officials have said that since the security crackdown in the Baghdad region began more than 12 weeks ago, Sunni insurgents have hit back with powerful, and extremely deadly, car bombs that often cause more casualties than the types of attacks used previously.

The car bomb attack occurred about 7:45 p.m. Tuesday in the village of Abu Saydah in the volatile Diyala province, local police said, giving the casualty toll. The wounded were taken to hospitals in nearby Muqdadiyah and the main Shia district of Sadr City in Baghdad.

Hospital officials and victims said it appeared chlorine gas was used in the attack as many of the wounded were having difficulty breathing and had their sight affected. But officials at the provincial police's joint coordination centre denied that toxic gas was involved.

One man had a white cloth across his eyes as he lay in his hospital bed; others were bandaged from head to toe.

Abu Saydah is a mainly Shia village about 25 miles northeast of the Sunni insurgent stronghold of Baqouba, the capital of the province that has seen a recent spike in violence largely blamed on militants who fled Baghdad ahead of a US-Iraqi security crackdown.

Kadim Hussein, a 45-year-old farmer who was taken to the Imam Ali hospital in Sadr City, claimed the hospitals in Baqouba would only accept Sunnis.

"My eyes became puffy due the chlorine gas that was packed in the car bomb," he said, adding he also had difficulties breathing. "Also I had many pieces of shrapnel in my chest and right shoulder."

A hospital official said the facility had received three bodies and 11 of those wounded who all showed symptoms of chlorine poisoning. The official spoke on condition of anonymity because he wasn't authorized to speak to the media.

Diyala province with its mixed Shia and Sunni Muslim population has been the scene of frequent violence of a sectarian nature as well as attacks by anti-US insurgents.