Abu Musab al-Zarqawi's successor as head of al-Qaeda in Iraq vowed today to revenge the terror chief's killing, as President Bush made a surprise visit to Baghdad.

Mr Bush arrived unannounced in the fortified Green Zone on a trip that was kept secret even from Nouri al-Maliki, the new Iraqi Prime Minister, who only found out that Mr Bush was in Iraq five minutes before they met.

The US president had been due to hold a teleconference with Mr al-Maliki and his ministers from Camp David, but secretly left the Maryland retreat last night to fly to Baghdad. After arriving at Baghdad International Airport, he transferred to a Blackhawk helicopter for the eight-minute flight to the Green Zone.

It was only his second trip to Iraq since the invasion and the first since he shared a Thanksgiving meal with soldiers at Baghdad airport in November 2003.

Mr Bush met Mr al-Maliki at a former Saddam Hussein palace now used as the US Embassy. Seated alongside him, he said: "It’s our interest that Iraq succeed... I’ve come to not only look you in the eye," Bush told al-Maliki. "I’ve also come to tell you that when America gives its word, it keeps its word."

White House officials said that Mr Bush would also visit US troops on a vist expected to last around five hours.

Mr Bush's visit comes less than a week after a US air strike killed al-Zarqawi, the Jordanian militant blamed for many of the worst atrocities of the Iraqi insurgency, including numerous car bomb attacks, mass executions and the murder of Western hostages.

Al-Zarqawi's successor, Abu Hamza al-Muhajir, vowed in a statement posted on an Islamist website today that his predecessor would be avenged. "The day of vengeance is near and your strong towers in the Green Zone will not protect you," said the statement, whose authenticity could not be verified.

Mr Bush has already said that the new al-Qaeda chief would be "on our list" of targets.

As Mr Bush, accompanied by a small group of aides and reporters, flew in, a wave of bombings hit the oil city of Kirkuk, killing 14 people, in what was seen as a bid by al-Qaeda to show that al-Zarqawi's death would not stop its campaign of violence.

In one of the Kirkuk attacks, a car bomb exploded outside the house of a senior police officer, seriously wounding him and killing one of his bodyguard. As police and U.S. forces gathered in the area, a roadside bomb exploded, killing 10 civilians, in a common tactic by Sunni Arab insurgents seeking to topple the US-backed government.

"The terrorists want to send a message that they are staying active despite the fact that Zarqawi was killed," said Rizgar Ali, head of Kirkuk’s governing council

A senior Iraqi defence ministry official told Reuters more than 40,000 Iraqi and US forces backed by tanks would launch an anti-insurgent crackdown in Baghdad tomorrow, in what would be one of the biggest such operations since the 2003 war.

"Armoured personnel carriers and tanks will be used. We will depend on intelligence to find suspects," said Major General Abdel Aziz Mohammed.

Mr Bush's previous trip to Baghdad was confined to the airport and limited to several hours after dark - and was not publicised until he was safely in the air heading home.

This time Mr Bush landed at Baghdad airport in temperatures over 100F (38C) before heading to the Green Zone, the Camp David ruse serving as an elaborate ploy to safeguard his security.

The American death toll in Iraq, where there are some 130,000 US troops serving, is fast approaching 2,500 and Mr Bush's visit was seen as an attempt to take advantage of the death of al-Zarqawi to shed a more positive light on the conflict. Recent polls suggest that only one-third of voters support Mr Bush's handling of the war, threatening the Republicans with severe losses in mid-term elections later this year.