He made the surprise visit after US President George W Bush said yesterday that he plans to boost the size of the army and Marine Corps in Iraq.

Gates and General Peter Pace, chairman of the Joint Chiefs of Staff, were scheduled to meet American commanders and Iraqi political leaders, including Prime Minister Nuri al-Maliki.

"The whole purpose is to go out, listen to the commanders, talk to the Iraqis and see what I can learn," Gates, sworn into office two days ago, told reporters on the way to Baghdad.

The former CIA director's visit follows a Pentagon report that said violence in Iraq was at an all-time high. At least 15 people were killed in yesterday's violence.

A suicide attacker ploughed a car bomb into a police checkpoint protecting Baghdad University yesterday, killing at least 11 people and wounding 30 more, medical officials said.

Shortly afterwards another car bomb killed four people and wounded seven when it exploded outside a government passport office in the Kasr neighbourhood in the north of the city, according to a security official.

Medics at the war-torn Iraqi capital's Yarmuk and Ibn Nafis hospitals said most of the casualties in the campus attack were students, while a security official said three national police officers were killed and seven wounded.

"People were hopping over and treading on bodies as they scattered to get away," said a young man who fled the scene.

The explosion took place on a major road leading through the Karrada area, a once prosperous part of Baghdad which was known as one of the most mixed areas of the city but is now increasingly dominated by Shiites.

Baghdad's universities have come increasingly under attack from Sunni extremist groups, and last week an Internet statement purportedly from the Ansar al-Sunna militant organisation explicitly threatened students.

Both attacks came as Baghdad remained trapped in a vicious cycle of sectarian killings led by warring Sunni insurgent groups and Shiite militias.

And, in a turnaround from past upbeat assessments on the Iraq war, Bush told the Washington Post in an interview posted on the newspaper's Web site: "We're not winning, we're not losing."

He is facing mounting pressure to reduce America's military commitment in Iraq but has also said he wants to expand the overall size of the US armed forces.

White House spokesman Tony Snow said the Iraq troop increase "is something that's being explored" amid media reports that the president might add tens of thousands of US soldiers to help quell what the Pentagon now warns is the worst violence on record.In an interview in the Wednesday edition of The Washington Post, Bush for the first time admitted that the United states was not winning in Iraq.

"We're not winning, we're not losing," Bush said in a marked reversal from a remark he made before the November 7 Congressional election that "Absolutely, we're winning."

Bush's Republican Party lost control of the US Congress in that election to opposition Democrats largely due to voter anger over the war in Iraq.

Bush made no specific mention of the Iraq troop increase except to say that "all options are viable," but told the Post that he ordered the new defense secretary, Robert Gates, to develop a plan to expand the US army and Marine Corps.

"I'm inclined to believe that we do need to increase our troops, the army, the Marines," he said.

Bush declined to put a number on the increase, and disputed former US secretary of state Colin Powell's assertion over the weekend that "the active army is about broken" due to strains from Iraq and Afghanistan.

"I haven't heard the work 'broken,' but I've the word 'stressed,'" said the president, who told the Post that more ground forces were required to fight a global war on terrorism.

"It is an accurate reflection that this ideological war we're in is going to last for a while, and that we're going to need a military that's capable of being able to sustain our efforts and to help us achieve peace," he said.

"We need to reset our military. There's no question the military has been used a lot," said Bush.