US authorities are looking into whether al Qaeda in Iraq leader Abu Musab al-Zarqawi was killed in a gunfight in the northern Iraqi city of Mosul, a US official said on Sunday, but a White House spokesman said that was “highly unlikely.” If Zarqawi were found to be among the dead, the United States would view it as a major advance in its efforts to quell a bloody insurgency in Iraq, at a time of acrimonious debate over both the origins and progress of the war.

Eight insurgents, including a woman, were killed on Saturday in clashes between joint US-Iraqi forces and gunmen occupying a house in Mosul, police said. Four more insurgents were arrested. “Efforts are under way to determine whether Zarqawi was among those killed,” the US official said in Washington, speaking on condition of anonymity. He provided no additional information. White House spokesman Trent Duffy, travelling with President George W. Bush in Asia, said it was “highly unlikely and not credible” that Zarqawi was among the dead in Mosul.

A Pentagon spokesman said he had no information on Zarqawi, the head of al Qaeda’s Iraq wing. The Jordanian-born Zarqawi is Washington’s most wanted man in Iraq, with a $25 million US reward on his head. Bush, facing waning public support for the war in Iraq as casualties mount, has countered critics demanding to know when the more than 150,000 troops in Iraq will come home by saying he will set no timetable for a withdrawal. Bush says such a timetable would be a green light for insurgents.

Over the past week, his administration has launched a counter-offensive against critics accusing the White House of misusing intelligence about the threat of Iraqi weapons of mass destruction to justify the invasion. The Bush administration calls the Iraq war a central front on the war on terrorism it declared after the September 11, 2001, attacks on the United States.

Said to be in his late 30s, Zarqawi remains a mysterious figure. Some posters show him in glasses, looking like an accountant, others as a tough-looking man in a black skullcap. When an Islamist Web site showed a video last year of a man severing the head of American hostage Nicholas Berg, the CIA said Zarqawi was probably the one wielding the knife.