A roadside bomb has killed four contractors and wounded eight others in northern Iraq, the US military said.

The bomb struck a convoy carrying contractors near Mosul on Monday, the military said in a statement released on Tuesday.

It is not known whether the contractors were Iraqis or foreigners.

US commanders have described Mosul as al-Qaeda in Iraq's last urban stronghold. Iraqi troops backed by the US military launched a major offensive in the city in May.

The blast happened the same day that Iraq's prime minister for the first time publicly called for a US troop withdrawal timetable.

Withdrawal talks

Nuri al-Maliki said on Monday that a military agreement the two countries are negotiating should include provisions for the withdrawal of American troops.

With the country facing its lowest levels of violence in four years, Iraq's government has felt increasingly confident about its authority.

The White House said it did not believe al-Maliki was proposing a rigid timeline for US troop withdrawals.

"Negotiations and discussions are ongoing every day," Gordon Johndroe, a White House spokesman, said on Tuesday in Japan, where George Bush, the US president, was attending the Group of Eight summit.

"It is important to understand that these are not talks on a hard date for a withdrawal."