Iraq's prime minister announced Friday that the government was launching a major offensive against Al Qaeda in the northern city of Mosul after two days of deadly bombings that killed nearly 40 people.

He promised the fight "will be decisive."

The announcement by Prime Minister Nouri al-Maliki came after warnings by the U.S. military that Mosul was the last major city where Al Qaeda maintains a strong presence after largely being driven from Baghdad and other major population centers.

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Al-Maliki said the government was sending troops to Mosul and an operations room had been established to fight the insurgents.

"Today, the troops have moved to Mosul ... and the fight there will be decisive," al-Maliki said during an address in the Shiite holy city of Karbala.

He did not say how many troops were being sent or provide more details in his wide-ranging speech, an apparent attempt to show his beleaguered administration was assuming control of the situation in Mosul with the U.S. military in the background.

Residents reported no immediate sign of stepped up security in the area and it was unclear when the offensive would begin.

The recent violence in Mosul, 225 miles northwest of Baghdad, began Wednesday when an abandoned apartment building believed to be used as a bomb-making factory exploded after the Iraqi army arrived to investigate tips about a weapons cache.

At least 34 people were killed and 224 wounded when the blast tore through surrounding houses in the Zanjili neighborhood, a poverty-ridden district on the west bank of the Tigris River.

Residents reportedly back Al Qaeda as a firewall against Kurdish influence in the city and the ineffective provincial government.

A suicide bomber then killed a police chief and two other officers Thursday as they toured the devastation from the previous day. Residents with insurgent sympathies taunted the chief moments before the attack.

Al-Maliki traveled to Karbala, 50 miles south of Baghdad, a day after a roadside bomb targeted a senior aide of Iraq's Shiite spiritual leader Grand Ayatollah Ali al-Sistani in the city.

The aide, Abdul-Mahdi al-Karbalai, escaped with a wound to the arm, but two of his bodyguards were killed and two were wounded, according to local police. Al-Maliki met with the white-turbaned cleric, who wore a bandage on his right forearm.

There have been several assassination attempts against al-Sistani's followers in recent months as internal Shiite rivalries increased in oil-rich southern Iraq, which also is home to some of the majority sect's most sacred shrines.

Al-Maliki, a Shiite, called the attack against al-Karbalai "a criminal act ... and a conspiracy by the followers of the former regime or by foreign intelligence services," although he was not more specific.

"Those who think they can bring back the past era are deadly wrong and ignorant," he said.

Al Qaeda and its supporters would find themselves without a major base of operations if ousted from Mosul, which occupies transport crossroads between Baghdad, Syria and other points.

But a drawn-out fight could serve to rally insurgents and expose potential security weaknesses where U.S. troops are thin and Iraqi forces must take a front-line role.

"Mosul will continue to be a center of influence for, center of gravity for, Al Qaeda," said U.S. military spokesman Rear Adm. Gregory Smith earlier this week, calling it a hub for both insurgent financing and foreign fighters.

"The flow through Mosul is critical for Al Qaeda in Iraq," he added.

But Mosul only gained this role after extremists were put on the run.

Al Qaeda first started to lose its footholds in the western Anbar province after Sunni tribes turned against them and joined the U.S.-led fight. The military successes then began to pile up in Baghdad and other central regions — forcing many insurgents to seek new havens in the north.

The centerpiece of the shift is Mosul, where insurgents are believed to use the cover of sprawling sheep and produce markets to smuggle cash, weapons and foreign fighters from nearby Syria.

With the northward migration, U.S. officers have sought — but not received — additional forces for Mosul. They also see no immediate signs of a Sunni groundswell as in Anbar and other places.