A senior U.S. military official said Friday that the leader of Al Qaeda in Iraq, Abu Ayyub al-Masri, has not been arrested.

"He has not been detained," the official told Reuters, without giving further details.

Iraqi authorities on Thursday had said Iraqi police commandos captured the leader of Al Qaeda in Iraq in a raid in the northern city of Mosul, in what would be significant blow to the Sunni insurgency in its last urban stronghold.

News of the reported capture was trumpeted by Iraqi state television and numerous Iraqi government and provincial officials.

Iraqi Defense Ministry spokesman Mohammed al-Askari said late Thursday that the arrest was made in Mosul, where insurgents have sought to establish a foothold after being widely uprooted from Baghdad and surrounding areas last year.

Interior Ministry spokesman Maj. Gen. Abdul-Karim Khalaf said the arrest occurred "at midnight and during the primary investigations he admitted that he is Abu Hamza Al-Muhajir."

There have been false alarms in the past about al-Masri. At least twice — in 2006 and May 2007 — reports circulated that al-Masri was dead, but they were later proved wrong.

Khalaf told the Iraqi state television that al-Masri was arrested during a police raid in Mosul, but gave no other details. "Now a broader investigation of him is being conducted," he said.

Mosul was considered the last important urban staging ground for Al Qaeda in Iraqi and allied groups after losing strongholds in Baghdad and other areas during the U.S. troop "surge" last year.

In January, Iraq's prime minister Nouri al-Maliki promised his military were preparing for a "decisive" showdown with insurgents in Mosul, about 225 miles northwest of Baghdad. But no major offensives have been mounted even as Al Qaeda in Iraq tried to exert its influence in Iraq's third-largest city through attacks and intimidation.

Al-Masri is a prized target for Iraqi commanders, who have led operations in the Mosul area and have sought to counter worries that Iraqi forces lack the training and discipline to wage a head-on fight against insurgents.

However, Al Qaeda in Iraq has maintained its long-term ability to wage suicide attacks and other strikes even after the capture and deaths of its leaders. Al-Masri took over Al Qaeda in Iraq after its leader, Abu Musab al-Zarqawi, was killed June 7, 2006 in a U.S. airstrike northeast of Baghdad.

The pace of insurgent attacks continued unabated as al-Masri took charge.

Al-Masri has been in contact with some of bin Laden's top lieutenants.

U.S. officials said al-Masri — whose name means "The Egyptian" in Arabic — joined Al Qaeda camps in Afghanistan in the late 1990s and trained as a car bombing expert before traveling to Iraq after the U.S.-led invasion in 2003.

The U.S. military also described al-Masri as a previous member of the extremist Islamic Jihad in Egypt and a protege of Ayman al-Zawahri, who became bin Laden's No. 2 after the group joined with Al Qaeda in 1998.

The Islamic State of Iraq, an umbrella organization that includes Al Qaeda in Iraq, last year announced an "Islamic Cabinet" for Iraq and named al-Masri as "minister of war." The U.S. military had put a $5 million bounty for al-Masri.

Meanwhile, the U.S. military on Friday said U.S. soldiers killed six Shiite extremists, who attacked U.S. forces with shoulder fired rockets and small arms, in several clashes in Baghdad's Shiite militia stronghold of Sadr City on Thursday.

The reported arrest of al-Masri has also turned attention back to the Sunni insurgency after weeks of battles with Shiite militias.

On Thursday, government envoys set strict demands for Shiite militias to end their battles against U.S.-led forces in Baghdad. But it was unlikely that militiamen would abide by the government conditions to lay down their arms.

But the government outreach to representatives of anti-American cleric Muqtada al-Sadr — who controls the powerful Mahdi Army militia — underscored the worries about a mounting humanitarian and political crises for Iraq's leadership if the fighting spreads.

Thousands of civilians already have fled their homes in Sadr City — home to nearly 40 percent of Baghdad's population — and aid groups say some areas are desperately short of food and medicine after seven weeks of street battles.

The latest conflict flared in late March after Prime Minister Nouri al-Maliki ordered a crackdown on armed Shiite factions in the southern city of Basra, the nation's second-largest urban area. Mahdi fighters quickly rose up in Basra and Sadr City, their stronghold in Baghdad.