Baghdad, Iraq ? U.S. troops have sharply increased patrols in Baghdad since the spike in sectarian violence, a U.S. general said Thursday, raising questions about the capabilities of Iraqi forces. A car bomb killed at least 15 people in a Shiite area of the capital Thursday.

At least 21 other people, including an American soldier and seven members of a Sunni family, were killed Thursday.

With sectarian violence on the rise in Baghdad, the U.S. command boosted the number of armed patrols in the capital from 12,000 in February to 20,000 since the beginning of March, Maj. Gen. Rick Lynch told reporters.

Lynch said the increase provides a “more visible presence for the security forces in the streets of Baghdad,” which he said insurgents consider their “center of gravity” to stop formation of a new unity government.

“We’re taking the fight to the enemy specifically in Baghdad with the presence we have on the ground,” Lynch said.

An Iraqi man mourns a victim of a Wednesday night car bomb, which exploded as people were leaving a mosque near Baqouba, after bringing his relative to be buried Thursday April 13, 2006 in Najaf, 160 km (100 miles) northeast of Baghdad, Iraq. A car bomb exploded Wednesday night as worshippers were leaving a Shiite mosque in a village near Baqouba, northeast of Baghdad, killing at least 20 people and injuring 32 others, police said.

Tit-for-tat killings between Shiites and Sunnis soared after the Feb. 22 bombing of a major Shiite shrine in Samarra, triggering reprisal attacks against Sunni mosques and clerics. Violence was worse in religiously mixed areas of Baghdad, forcing the Americans to return to neighborhoods such as Shula that had been turned over to the Iraqis.

That casts doubt on the capability of Iraqi forces to deal with sectarian violence, despite assurances from American officials that the new army and police forces were gaining steadily in professional skills.

The renewed American presence has not been enough to stop the carnage. The car bomb exploded in a vegetable market in Shula packed with shoppers buying food for their evening meals, police said. At least 15 people were killed and 22 were wounded. Last week, a car bomb injured 13 people in the same neighborhood.

A roadside bomb Thursday killed a U.S. soldier southwest of Baghdad, the military said. The U.S. command also reported that a Marine died Wednesday of wounds suffered in hostile action near Baghdad.