Similar reports in the past proved inaccurate. U.S. Ambassador Ryan Crocker told reporters that American authorities in Baghdad were seeking more information.

An umbrella organization of Iraqi insurgent groups denied the al-Qaida leader had been killed, saying he was alive and safe, according to an Internet statement.

"The Islamic State of Iraq reassures the Ummah (nation) that Sheik Abu Hamza al-Muhajer, God protect him, is alive and he is still fighting the enemy of God," the umbrella group said on a Web site commonly used by insurgents.

A series of reports Tuesday said Abu Hamza al-Muhajer — whom U.S. and Iraqi forces identify by another pseudonym, Abu Ayyub al-Masri — had been killed, either by rivals in al-Qaida or Sunni tribesmen who have turned against al-Qaida.

A Pentagon spokesman, Col. Gary Keck, said he was aware of the reports from Iraq but had no confirmation.

"U.S. forces are working with Iraqi officials to determine if this is true," he said, adding that he did not know whether U.S. forces were at the site of the alleged killing.

In recent months, divisions among Sunni insurgent groups have sharpened, in part because of al-Qaida's attempt to dominate the "resistance," impose a harsh brand of Islam on ordinary people and use foreign fighters, U.S. officials say.

More than 200 Sunni Arab sheiks in Anbar province have decided to form a political party to oppose al-Qaida. Clashes have erupted in three Sunni provinces between al-Qaida and other insurgent groups, notably the nationalist 1920 Revolution Brigades, U.S. officers say.

Iraqi officials released conflicting accounts of when and where al-Masri was purportedly killed, and who was supposed to have killed him. It was also unclear whether Iraqi authorities had the body.

Chief government spokesman Ali al-Dabbagh Al-Dabbagh told Al-Arabiya television that the report of al-Masri's death was based on "intelligence information," adding that "DNA tests should be done and we have to bring someone to identify the body."

But he refused to say unequivocally whether Iraqi security forces had the body, citing security restrictions.

The Interior Ministry spokesman, Brig. Gen. Abdul-Karim Khalaf, told state television that authorities did not have absolute confirmation al-Masri was dead but said reports indicated he was killed by fellow al-Qaida members in an ambush at the Safi bridge north of Baghdad.

"Sources of the Interior Ministry witnessed the killing of this criminal," Khalaf said.

Deputy Prime Minister Barham Saleh told The Associated Press that al-Masri was believed to have been killed Monday in the Taji area north of Baghdad.

"Preliminary reports said he was killed yesterday in Taji area in a battle involving a couple of insurgent groups, possibly some tribal people who have problems with al-Qaida. These reports have to be confirmed."

A U.S. spokesman, Lt. Col. Christopher Garver, said the U.S. command was looking into the reports.

"Obviously I hope it's true," Garver said, pointing out that previous Iraqi claims had proven false. "We want to be very careful before we confirm or deny anything like that."

Al-Masri, an Egyptian militant, took over leadership of the terror network and was endorsed by Osama bin Laden after Abu Musab al-Zarqawi was killed last June by a U.S. airstrike in Diyala province.

During a teleconference with reporters in Washington, the U.S. ambassador said al-Masri's death would be a positive development but played down suggestions it would have any immediate impact on al-Qaida activity in Iraq.

"Clearly taking a major terrorist off the battlefield is an important thing and if we can confirm it, if this did happen, without question it would be a significant and positive development," Crocker said. "That said, I would not expect it to in any way bring to end al-Qaida's activities in Iraq."

The Islamic State of Iraq, the umbrella organization that includes al-Qaida in Iraq, said it published the denial of al-Masri's death "to reassure the hearts of Muslims."

"We warn that the enemy is still playing the tune of trying to break the ranks of Mujahedeen in Iraq; this is unattainable and it will not take place, God willing," the group's statement said.