BAGHDAD - The Iraqi government said yesterday that it was investigating reports from "reliable sources" that the leader of al Qaeda in Iraq had been killed in an ambush by a rival insurgent faction.

In recent months, some Sunni tribes have allied with government forces to fight al Qaeda in Iraq. The sheiks, some of whom have been involved in the anti-American resistance, have opposed al Qaeda's efforts to impose a fundamentalist Islamic state in Anbar province and to dominate the insurgency.

Iraqi officials, desperate to show gains against a relentless insurgency, have in the past trumpeted the killing or capture of major insurgent leaders only to retract the claims later.

U.S. authorities urged caution and warned that even if the claim were true, the death of the shadowy Abu Ayyub al-Masri would likely not spell the end of the terror movement in Iraq. *