BAGHDAD - A suicide bomber in police uniform killed the police chief of the northern Iraqi city of Mosul on Thursday as he toured the scene of a blast a day earlier in which at least 20 people died, police said.

Rescuers were still digging through the rubble of Wednesday's explosion in search of survivors when the attacker blew up next to Mosul police Brigadier-General Salih Mohammed Hasan and his bodyguards, the army said.

Army Major-General Riyadh Jalal Tawfiq said the suicide bomber walked up to Hasan as he toured the site and detonated a vest packed with explosives.

Brigadier-General Abdul Kareem al-Jubouri, head of operations in the Mosul police command, said Hasan died of his wounds as he was being rushed for emergency treatment.

One of Hasan's bodyguards and a policeman were also killed and another five wounded. Police said an Iraqi journalist working for the Chinese state news agency Xinhua was among the wounded, although no more information was immediately available.

The U.S. military said it was checking Iraqi media reports that a U.S. soldier had been wounded.

U.S. commanders have identified Mosul, Iraq's third-largest city, as al Qaeda's last major urban stronghold in Iraq after its fighters were driven out of western Anbar province and from around Baghdad during security crackdowns last year.

Mosul officials earlier imposed an indefinite curfew after Wednesday's massive blast, blamed by U.S. military commanders on al Qaeda, killed 20 people and wounded 150 in Mosul, northern Iraq's largest city.

Senior Iraqi security forces officials, who refused to be identified, feared the death toll from Wednesday's blast would rise as rescuers picked through rubble in the ethnically and religiously mixed city 390 km (240 miles) north of Baghdad.

ARMS STORE

Women and children were among the victims, officials said. Heavy equipment was being used to clear the debris and the U.S. military has sent medical teams to Mosul to help treat the scores of wounded.

The explosion, in an unoccupied, three-storey house used by militants to store weapons and tonnes of explosives, destroyed or badly damaged 35 nearby homes.

"This is a stark example of al Qaeda's disregard for the citizens of Iraq," Major-General Mark Hertling, commander of U.S. forces in northern Iraq, said in a statement.

"It also highlights their willingness to risk the lives of innocent civilians by storing weapons in civilian homes."

Hertling said on Wednesday it appeared that Iraqi soldiers had triggered a massive secondary explosion when they detonated a roadside bomb they found near the building.

He said explosives experts at the scene estimated 15 tonnes of ordnance had been hidden there.

Witnesses described the blast, which sent a huge plume of smoke rising above Mosul, as one of the biggest explosions ever heard in the city.

Mosul is the capital of Nineveh province, one of Iraq's northern regions where U.S. and Iraqi forces this year have launched offensives against Sunni Islamist al Qaeda fighters who are most often blamed for large-scale bombings in Iraq.

Violence has fallen sharply across the country, with overall attacks down by 60 percent since last June.

U.S. and Iraqi officials attribute the decline in violence to the deployment of an extra 30,000 U.S. troops and the growing use of mainly Sunni Arab neighbourhood police units in areas where local citizens have turned against al Qaeda.

In other violence on Thursday, two policemen were killed and three others wounded by a roadside bomb in central Baghdad's Karrada district, police said.