U.S. ambassador to Iraq Christopher Hill was unhurt when a roadside bomb blasted his convoy in the south of the country, USA Today reported on Monday.

The American daily said its reporter was a few minutes behind Hill in a separate convoy on Sunday when the bomb struck in Nassiriya, 300 km (185 miles) southeast of Baghdad. No one was hurt in the bombing, it said.

"There was a bang and we went through a thick cloud of smoke," Hill told the reporter afterwards. "We are all fine."

The U.S. embassy could not be immediately reached.

Despite a sharp drop in violence across Iraq in the past 18 months, militants are still able to carry out frequent bomb attacks. U.S. forces withdrew from Iraqi cities at the end of last month, raising doubts about whether Iraq's own fledgling forces are up to the job of keeping the country safe.

Iraq is littered with munitions dumps from the time of Saddam Hussein, many of them left unguarded by U.S. forces after the 2003 invasion to oust him. Militants have since then had a steady supply of explosives with which to fashion home made bombs that can easily be concealed in pot holes or trash piles.

Largely Shi'ite southern Iraq has been mostly quiet since Iraqi forces launched a crackdown on militants loyal to anti-American Shi'ite cleric Moqtada al-Sadr last year. The U.S. military usually blames the sporadic attacks that do happen there on militias with links to Iran, a charge Tehran denies.