A suicide bomber killed an Iraqi police chief and two other officers Thursday after they toured the site of the wreckage of a deadly blast a day earlier that devastated a predominantly Sunni neighborhood in the volatile northern city of Mosul, officials said.

The attack compounded the tragedy after an explosion on Wednesday collapsed a three-story apartment building and ravaged adjacent houses just minutes after the Iraqi army arrived to investigate tips about a weapons cache. Police on Thursday raised the casualty toll from that blast to at least 18 killed and 146 wounded.

The bomber on Thursday was wearing an explosives vest under an Iraqi police uniform when he struck, killing Brig. Gen. Saleh Mohammed Hassan, the director of police for Ninevah province, the U.S. military said, adding that a U.S. soldier and an Iraqi soldier also were wounded.

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Police spokesman Brig. Gen. Saeed al-Jubouri said Hassan was attacked after gunmen ambushed the blast site, sparking clashes that lasted for about 15 minutes. The bomber moved toward Hassan's car as it was preparing to flee the area, al-Jubouri said.

The military said the Iraqi army was securing the area.

A bulldozer worked through the night to clear the debris, with vehicles providing light as dozens of people watched on the rim of a massive crater that was left by the blast, according to footage from the local Mosuliyah TV station.

The TV footage showed one woman looking stunned as she held a bandage to her face in an emergency room and doctors rushed to treat a man whose face was bloodied.

The explosion occurred after the army received calls that insurgents were using the vacant building as a shelter and a bomb-making factory, according to al-Jubouri, the police spokesman.

"Everything on the kitchen shelves fell on me, and I started to scream for help until my husband came and took me to the hospital," said 25-year-old Um Mohammed, who was treated for wounds to her head, legs and left hand.

Her husband, 32-year-old taxi driver Abu Mohammed, escaped with minor injuries to his hands.

"I was standing near my house behind the exploded building when a very loud blast took place, and the smoke covered the whole area," he said. "I was confused and went inside my house to search for my wife. Everything in the house was turned upside down. I saw my wife lying on the ground and I carried her to my car and headed to the hospital. What has happened is a disaster."

Duraid Kashmola, the governor of Ninevah province, of which Mosul is the capital, imposed an indefinite curfew in the city's downtown area following the clashes.

American and Iraqi forces have been on the offensive against militants in and around Baghdad, but Mosul -- Iraq's third-largest city some 360 kilometers (225 miles) northwest of the capital -- continues to be a center of gravity for al-Qaida in Iraq and other insurgents, according to the U.S. military.

The blast in Mosul was the latest in a series of bombings across Iraq, including in some areas that have seen a relative calm recently with the security gains from U.S.-Iraqi operations and a Sunni revolt against al-Qaida in Iraq.

A roadside bomb also struck a police patrol Thursday in central Baghdad, killing two officers and wounding two others, along with three civilians, police said. The explosion occurred about 8 a.m. in the predominantly Shiite neighborhood of Karradah.

Mosul, a major transportation hub with highways leading west to Syria and south to Baghdad, is considered a crucial conduit in the flow of money and foreign fighters to support the insurgency. The military said earlier this week that it was the last urban center with a strong presence of al-Qaida in Iraq.

Attacks have persisted in recent months in northern Iraq even as violence has declined in Baghdad and other areas.

In a separate incident, a suicide car bomber targeted a police convoy near the northern city of Kirkuk on Wednesday, killing at least five civilians and wounding 11, police said.

In the capital, Baghdad, gunmen fired on Iraqi soldiers resting on the side of a highway, killing three and wounding at least one, according to police and the U.S. military. The attack in the heart of Baghdad provided a deadly example of the challenges facing the Iraqi forces as they work to take over security so U.S.-led troops can eventually go home.

Five militant Iraqi Sunni groups said in a joint statement posted on the Internet that they were stepping up attacks on American troops in Iraq in support of Palestinians in the Hamas-controlled Gaza Strip.

The statement announced the launching of what was described as the "Iraqi Resistance Campaign to Help Gaza" and accused U.S. President George W. Bush and Israeli Prime Minister Ehud Olmert of responsibility for the deteriorating situation in the coastal strip.