A car bomb has killed at least 15 people in a northern Iraqi city, on the day that Iraqi forces officially assume control of security in Baghdad and other urban areas.

Further details of the bombing in Kirkuk were not immediately available.

The Iraqi government declared Tuesday a "Day of National Sovereignty" following the withdrawal of U.S. combat troops from urban areas. Prime Minister Nouri al-Maliki reviewed members of the Iraqi military as a band played, and he called the day a "great victory."

While some Iraqis are joyous at the prospect of seeing U.S. combat troops leave, others are nervous about the change in security and fear an increase in militant attacks.

The withdrawal is part of U.S.-Iraq security deal that sets a timeline for the withdrawal of all U.S. forces by the end of 2011. For the time being, tens of thousands of U.S. troops will remain in the country to train and advise Iraqi forces, and to conduct combat duties outside cities.

Some information for this report was provided by AP and Reuters.