Thousands of Iraqi and US-led forces prepared to mount a major security crackdown in violent Baghdad on Wednesday, hours after a surprise visit by President George W. Bush to try to bolster Iraq's new government.

As Bush talked to Iraqi leaders in Baghdad's heavily fortified Green Zone on Tuesday, a Web statement said al Qaeda's new leader in Iraq had vowed to avenge the killing of his predecessor Abu Musab al-Zarqawi in a US air strike last week.

President Bush greets military personnel and embassy employees at the US Embassy after he arrived in Baghdad for a surprise visit to the city, June 13, 2006. [Reuters]

"The day of vengeance is near and your strong towers in the Green Zone will not protect you," said the statement, posted on an Internet site often used by Islamist militants and signed by the new leader Abu Hamza al-Muhajir.

US and Iraqi officials have hailed the killing of Zarqawi, a Sunni Arab like most guerrillas in Iraq, as a major blow to al Qaeda militants seeking to topple the Shi'ite-led government, while cautioning it will not bring an early end to bloodshed.

"There are going to be tough days ahead, and more sacrifice for Americans, as well as Iraqis," Bush told US troops.

"Our military will stay on the offensive. We will continue to hunt down people like Mr. Zarqawi, and bring them to justice," he said to applause.

At least 14 people were killed in a series of bombings in the northern city of Kirkuk on Tuesday.

US and Iraqi military commanders say they are focusing their new security campaign on the capital Baghdad, a city of seven million people and scene of daily carnage.

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