At least 15 people were killed and 75 wounded when a building used by militants to store weapons and tonnes of explosives blew up in the northern Iraqi city of Mosul today, security officials said.
They said women and children were among the victims after the blast, which damaged nearby homes, and heavy equipment had been brought in to search for people buried under rubble.
The blast was possibly detonated as police arrived to search the building following a tip from a detained militant.
Witnesses said it was one of the biggest explosions ever heard in ethnically and religiously mixed Mosul, 390 km (240 miles) north of Baghdad. They said a huge plume of smoke rose above the largest city in northern Iraq.
The explosion destroyed the unoccupied three-storey building, security officials said.
Mosul is the capital of Nineveh province, one of Iraq's northern regions where US 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.
The offensives were carried out after al Qaeda militants were squeezed from former strongholds in western Anbar province and areas around Baghdad by security crackdowns last year.
Brigadier-General Noor Addin Hussein, an Iraqi army brigade commander in Mosul, said a militant who had recently been detained had told authorities that the building contained tonnes of explosives, weapons and ammunition.
The building blew up when security forces approached it, Hussein said.
Brigadier-General Abdul Kareem al-Jubouri, head of the operations room in the Mosul police command, added: "It seems that when the insurgents discovered that police and army forces had reached the building, they detonated (the explosives)."
In another attack in northern Iraq, a suicide car bomb killed seven people and wounded 16 others about 40 km (25 miles) from the city of Kirkuk, police said.
Despite persistent bombings in northern Iraq, violence has fallen sharply across the country, with overall attacks down by 60 percent since last June.
US and Iraqi officials credit the deployment of an extra 30,000 US troops and the growing use of mainly Sunni Arab neighbourhood police units in areas where local citizens turned against al Qaeda for the drop in violence.
US soldiers backed by attack aircraft killed 20 suspected al-Qaeda fighters in raids in northern Iraq over the past two days, the US military said.
In the biggest operation, US ground troops seeking an al Qaeda network leader near the Diyala provincial capital Baquba called in air support after encountering a number of militants who took up "fighting positions".
"Responding in self-defence, the ground force called supporting aircraft to engage the hostile force, killing 10 terrorists," the US military said in a statement.
Another three were killed in a nearby building and weapons caches including machine guns, rocket-propelled grenades, roadside bombs, artillery and mortar rounds and other ammunition were also found, the military said.
Baquba, 65 km (40 miles) north of Baghdad, and Mosul have become the two biggest headaches for US and Iraqi security forces battling al-Qaeda, which the military calls the greatest threat to security in Iraq.