The withdrawals, which will most likely come from Baghdad and Anbar province, once main battlefields of the war, are the first step in keeping with President Barack Obama's pledge to end America's role in the war.

That would bring US troop levels to more than 120,000 - still a substantial force. As part of the drawdown, the US will turn over 74 facilities and areas under its control to the Iraqis by the end of March.

All 4,000 British soldiers remaining in Iraq are also scheduled to leave by September.

A US spokesman, Maj. Gen. David Perkins, said Iraq's security has "greatly improved, moving from a very unstable to a stable position." He said violence was at its lowest level since the summer of 2003.

Sunday's attack was the worst in Baghdad in months, injuring about 60 people beyond the dozens killed. But in a news conference hours later, Perkins described it as a sign that US-led coalition forces have militants on the run.

"We know that al-Qaida, although greatly reduced in capability and numbers, still is desperate to maintain relevance here in Iraq," Perkins said.

The suicide bomber detonated his explosives as he drove his motorcycle into a group of people, many of them police recruits, waiting near a side entrance of Baghdad's main police academy in a mainly Shiite area of the city.

The heavily fortified academy has been hit by several bombings. A Dec. 1 suicide bombing there killed at least 33 and wounded dozens, including four US soldiers and an Iraqi general.

Iraqi and US forces sealed off the scene Sunday, allowing only ambulances and fire engines to enter. Nervous Iraqi troops fired in the air to prevent onlookers and reporters from getting too close.

Haitham Fadhel said he was standing in one of three lines of recruits arriving for their first day of special guard training courses. He was knocked unconscious and wounded by shrapnel, but called himself lucky. Two friends were killed.

"We were feeling secure as we were waiting in a well-guarded area," said Fadhel, 24. "Before the explosion occurred I heard a loud shout saying, 'Stop, stop, where are you going?' Seconds later, a huge explosion shook the area."

He added, "I am just wondering how a big security breach can occur in such a secured area."

No group claimed responsibility for the blast, but suicide bombings are the signature attack of Sunni religious extremists, including al-Qaida.

Iraqi officials provided conflicting casualty tolls, as is common in the chaotic aftermath of bombings.

Three medical officials and one police officer in the area said 32 were killed, including 19 recruits, nine policemen and four traffic police, with about 60 others wounded.

An Interior Ministry official said the death toll was 28 and 57 were wounded. The officials spoke on condition of anonymity because they weren't authorized to release the information.

Extremists increasingly have targeted Iraqi forces as they take over the country's security so American troops can go home. Last week, President Obama announced the end of all combat missions in Iraq by the end of August 2010, leaving up to 50,000 US soldiers to train and assist Iraqi security forces. All US troops are to leave Iraq by the end of 2011.

Currently there are about 135,000 US troops in Iraq, and their withdrawal will be gradual at first to leave most in place for parliamentary elections at the end of this year. The 12,000 troops, which make up two of 14 combat brigades in Iraq, will not be replaced.

Still, troop levels in Iraq are more than double what the Obama administration is sending to Afghanistan, where as many as 55,000 soldiers will be stationed by this summer.

In an interview Sunday, Iraqi Prime Minister Nouri al-Maliki predicted the US withdrawal would be "responsible" and said Iraqi military and police still need weapons "in order to protect the internal security situation in the first place."

"We, the Iraqi government, feel that the Iraqi security forces are capable of filling any vacuum created by he withdrawal of the US forces," Maliki told Iraqi government television.