Iraqi officials said more than 40,000 Iraqi and US-led forces backed by tanks and armored vehicles would take part in the mission, in what would be one of the biggest such operations since the US-led invasion in 2003.

"It is an operation to step up pressure on al Qaeda in Baghdad," national security adviser Mowaffaq al-Rubaie told state television.

The clampdown would include increased checkpoints and patrols, focusing on the dangerous, mostly Sunni Dora and Adhamiya districts. Insurgents draw support from Iraq's minority Sunni community, once dominant under Saddam Hussein.

"There is no time limit for ending this operation because it is a strategic plan through which we are determined to impose order in tense areas," Major General Abdel Aziz Mohammed, a senior Defense Ministry official, told Reuters.

US and Iraqi forces have carried out several such operations in the past but have failed to stem the violence.

BOMBINGS

The authenticity of Tuesday's statement from al Qaeda in Iraq, which under Zarqawi's leadership was widely blamed for a campaign of beheadings and suicide bombings that killed hundreds, could not be verified.

It would be Muhajir's first public statement since being named the new leader of al Qaeda in Iraq, one of several insurgent groups.

Some analysts say Muhajir may be a nom de guerre for Egyptian militant Abu Ayyub al-Masri, who trained in Afghanistan, formed al Qaeda's first cell in Baghdad and is sought by the US military as a Zarqawi aide.

Security concerns meant Bush's journey to Baghdad, his second trip to the country since US-led forces ousted Saddam, was top secret and many of his own aides, and even Iraq's new Prime Minister Nuri al-Maliki, were kept in the dark.

"I have come to not only look you in the eye. I've also come to tell you that when America gives its word, it will keep its word," Bush told Maliki, a tough-talking Shi'ite Islamist whose self-styled national unity government took office last month.

Maliki said his government was determined to defeat the insurgents so U.S. and other forces could withdraw.

"God willing all the suffering will be over, all the soldiers will return to their countries with our gratitude, for what they have offered, the sacrifices," he said.

The US death toll in Iraq since the March 2003 invasion is approaching 2,500, and tens of thousands of Iraqis have died.

Opinion polls show US public unease over Iraq is growing in a congressional election year but Bush has resisted calls to set a timetable for withdrawal of some 130,000 US troops, saying any pullout is conditional on improvements on the ground.