Rescue crews were searching late into the night for victims under the debris of collapsed homes from the thunderous 4:30 p.m. explosion. Police said injured people were trapped under the collapsed ceilings and walls of their homes.
The blasts reinforced U.S. claims this week that Mosul, 360 kilometers (225 miles) northwest of Baghdad, is the only major Iraqi city with a strong al Qaeda presence. American and Iraqi forces have been on the offensive against the terror group in and around Baghdad, but Mosul continues to be a center of gravity for al Qaeda in Iraq, according to the military.
Iraq's third largest city, a major transportation hub with highways leading west to Syria and south to Baghdad, is the main conduit in the flow of blood money and foreign fighters.
Mosul's disaffected populations also provide fertile ground for al Qaeda among fellow Sunni Arabs. The terror group is apparently seeking to exploit ethnic tension between majority Sunnis and minority Kurds, who together form about 85 percent of the city's population of roughly 2 million.
Wednesday's explosion, in a Sunni neighborhood in western Mosul, came shortly after the army received calls that insurgents were using the vacant building as a shelter and a bomb-making factory, according to Brig. Saeed al-Jubouri, a police spokesman.
That raised the possibility that the terror group may have soldiers into a trap.
However, Brig. Abdul-Karim al-Jubouri, who heads security operations for the Mosul police, said authorities did not believe that was the case. He said that if it were a trap, insurgents would have waited for security forces to get inside the building to kill as many of them as possible. The bombs went off just after the army arrived, and no soldier was known to have been killed.
Also, he said, insurgents usually warn nearby residents to leave before a bombing. On Wednesday, they did not.
"The insurgents used the building to store weapons and bombs, and it seems they blew up the building after learning that Iraqi soldiers had discovered their plans," Abdul-Karim al-Jubouri said.
The bombings collapsed the three-story building and damaged dozens of old houses in the area, according to Saeed al-Jubouri. He said 17 civilians were killed and 134 injured.
One of the injured, 25-year-old Um Mohammed, was making dinner for her family.
"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," she said while being treated for wounds to her head, legs and left hand.
Her husband, 32-year-old taxi driver Abu Mohammed, escaped with only 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."
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, killing at least five civilians and wounding 11, police said.
In the capital, Baghdad, gunmen opened fire 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 stark challenges facing the Iraqi forces as they work to take over their own 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.
It described the American and Israeli leader as "war criminals," saying they sought to "cover up their dreadful failures in all fields." It did not provide details of the planned attacks on U.S. troops.
In another development, Iranian President Mahmoud Ahmadinejad has accepted an invitation to visit Iraq, but no date has been set, the Iraqi Foreign Ministry announced. It would be the first visit to Iraq by a top Iranian leader since the Islamic revolution of 1979.
Iran had no immediate word on the visit.
The two Muslim neighbors fought a ruinous eight-year war in the 1980s that left an estimated 1 million people killed or wounded. Relations have improved since the 2003 ouster of Saddam Hussein's Sunni-led regime.
In other developments in Iraq:
The measure calls for removing the three green stars that are thought to symbolize Saddam's now-dissolved Baath Party's objectives of unity, freedom and socialism and changing the calligraphy of the words "Allahu Akbar," or God is Great, in a symbolic break with the past.
The decision - which was approved by a show of hands, with 110 lawmakers of 165 present voting in favor - is temporary. A law to establish a new banner must be passed in one year.