The blast hit an Iraqi patrol in Sadr City at midday, scattering burning debris across a small bridge, witnesses said.

An Associated Press reporter traveling with US troops nearby said the explosion showered shrapnel across a joint US-Iraq security station 300 yards away. The partially shattered windshield of a car landed at the gates of the compound.

Police said at least 18 people were killed and 48 wounded.

Home to about 2.5 million of Baghdad's poorest residents, Sadr City is the base for fighters allied to anti-American cleric Muqtada al-Sadr. His militia, the Mahdi Army, has laid low in recent weeks during a US-Iraqi security crackdown under pressure from Iraqi Prime Minister Nouri al-Maliki.

Hours earlier, Iraqi special forces teams backed by US soldiers detained six suspects believed to be a rogue members of the Mahdi Army, the US military said in a statement.

The suspects were accused of coordinating and carrying out kidnappings and murders of Iraqi civilians, the statement said.

In central Baghdad, two mortars fell near Iraq's Foreign Ministry, where envoys gathered for an international conference on how to quell the violence and bolster Iraq's government. There were no reports of injuries, but smoke was visible from the meeting area.

Meanwhile, Iraqi officials said they were holding a top al-Qaida official, but not the terror mastermind Abu Omar al-Baghdadi who they believed was captured a day earlier.

"After preliminary investigations, it was proven that the arrested al-Qaida person is not Abu Omar al-Baghdadi, but, in fact, another important al-Qaida official," said Brig. Gen. Qassim al-Mousawi, an Iraqi military spokesman.

"Interrogations and investigations are still under way to get more information," he said.

Al-Mousawi declined to give the suspect's name on Saturday.

It was al-Mousawi who announced late Friday that al-Baghdadi had been captured. A senior adviser to the prime minister also had told the AP that al-Baghdadi had been taken into custody. The adviser spoke on condition of anonymity because he was not authorized to release the information.

Al-Mousawi said the suspect at first identified himself as al-Baghdadi, and that his identity was corroborated by another man captured with him. The reported arrest followed rumors this week that al-Baghdadi's brother had been arrested in a raid near Tikrit.

Almost nothing is known of al-Baghdadi, including his real name and what he looks like; his capture would be difficult for officials to verify.

He is believed to lead the shadowy Islamic State of Iraq, an al-Qaida-inspired group that challenged the authority of Iraq's elected government. He has also signed militant messages posted online, as the leader of the Mujahedeen Shura Council an umbrella group that includes al-Qaida in Iraq.

An alleged member of the Islamic State of Iraq was among 27 suspects detained in U.S. raids across Iraq overnight, the US military said.

One suspect was killed and 18 were detained in Taji, an area on the northern outskirts of Baghdad, the military said in a statement.

Eight suspects were captured in Mosul, 225 miles northwest of Baghdad, and one was detained in Ramadi,115 kilometers 70 miles west of Baghdad, it said.

Also yesterday, the US military said it was investigating the shooting of three Iraqis in Baghdad's Azamiyah neighborhood. American paratroopers fired on a vehicle when it failed to respond to warning signals, the military said in a statement. Three Iraqis were killed and three others were wounded in Friday's incident, it said.

In other violence, a roadside bomb killed three Iraqi policemen and wounded another Saturday in central Ramadi, police said.

Gunmen opened fire on Shia pilgrims in Latifiyah, about 20 miles south of Baghdad, police said. One person was killed and three were wounded. Later, two more pilgrims were killed in shootings in eastern Baghdad, police said.

The pilgrims were on their way back from a Shia shrine in Karbala, where millions of faithful were performing rites this weekend for Arbaeen, a holiday that marks the end of a 40-day mourning period after the death anniversary of the Prophet Muhammad's grandson.

Some 340 people, mostly Shiite pilgrims en route to Karbala, were killed in sectarian attacks this past week.

STOP IRAQ AS BATTLE FIELD, SAYS MALIKI
Reuters says from Baghdad: Iraq's prime minister urged regional rivals yesterday to stop using Iraq as a battlefield to fight out their proxy wars, addressing a meeting at which US officials sat down with adversaries from Iran and Syria.

The conference in Baghdad aims to stop sectarian warfare in Iraq and prevent the conflict's spread throughout the region.

In his opening speech, Prime Minister Nuri al-Maliki said all those with a stake in the peace of the Middle East should stand firm against terrorism in Iraq.

"We call on all to take moral responsibility by adopting a strong and clear stance against terrorism in Iraq and cooperate in stamping out forces of terror," Maliki said, according to the text of his speech to the opening session.

"This is an international epidemic, the price of which is being paid by the Iraqi people and our country is on the frontline of confrontation," he said.

Maliki said Iraq needed the support of its neighbors and the world in stemming the Shia-Sunni sectarian violence, which he said could spill over to other countries in the region.

"Confronting terrorism means halting any form of financial support and media or religious backing, as well as logistical support and the flow of arms and men who transform themselves into bombs that kill our children, women and elders, and destroy our mosques and churches."

Iraq called the meeting to rally regional support to stop sectarian violence that threatens to tear the country apart, has killed tens of thousands and driven some 2 million abroad since a US-led invasion four years ago toppled Saddam Hussein.

Washington has accused Iran and Syria of fomenting violence in Iraq, charges both countries deny. Security officials in the region say Sunni extremists from neighboring Saudi Arabia and Syria are also entering Iraq.

Iran is a key ally of the Shia majority in Iraq, while Saudi Arabia and other Sunni Arab states have been traditional allies of the Sunni minority.

US Ambassador Zalmay Khalilzad urged Iraq's neighbors to do more to stop the flow of fighters, weapons and sectarian propaganda contributing to the violence, saying the future of Iraq and the Middle East was the defining issue of the moment.

"No country represented at the table would benefit from a disintegrated Iraq; indeed, all would suffer badly," he said. He hoped their presence indicated they were "ready to take concrete, constructive actions" to support Iraq.