BAGHDAD, Iraq — A car bomb exploded today in Baghdad's biggest Shiite district, killing seven people and injuring 15, officials said. Iraq's prime minister insisted that Iraqi forces were ready to take over security duties in most provinces despite rising violence.

U.S. officials confirmed that the number of roadside bombs directed against U.S. and Iraqi forces increased sharply last month, dramatizing the threat posed by the Sunni-led insurgency despite attention directed to sectarian violence in the capital.

The parked car exploded a little after noon near a market in Sadr City, inflicting the casualties and damaging many shops, said police Lt. Adil Salih.

The Iraqi army general command said in a statement that seven people were killed and 15 injured.

Residents said the number of casualties was relatively low because most people had finished their shopping early to escape the 120 degree heat that was forecast for Baghdad today.

Sadr City, a predominantly Shiite neighborhood, is one of the most tightly secured areas in Baghdad, patrolled by police as well as members of the Mahdi Army militia of the anti-U.S. cleric Muqtada al-Sadr.

The security is to prevent attacks by Sunni insurgents, but the latest attack demonstrates the difficulties of controlling the seething sectarian violence, which has risen steadily since the Feb. 22 explosion at a Shiite shrine in Samarra.