BAGHDAD — American and Iraqi troops moved against Al Qaeda on two separate fronts yesterday, with house-to-house searches in Mosul and an operation in the desert to stanch the flow of insurgents and weapons to that northern city.

With the new sweep, Prime Minister al-Maliki, is aiming to put down Sunni extremists after launching two other major offensives elsewhere in as many months targeting Shiite militants. Mosul, a key transport crossroads between Baghdad, Syria and other points, is considered the last major urban base of Al Qaeda in Iraq after the group lost strongholds in western Anbar province.

American-backed Iraqi troops searched homes and the U.S. military announced that the forces in Mosul captured a suspected Al Qaeda figure involved in organizing car bombings and smuggling foreign fighters into the country.

There were no reported clashes during the searches in known Al Qaeda strongholds in the western and eastern parts of Mosul, Iraq’s third largest city, where insurgents are believed to use the cover of sheep and produce markets to smuggle cash, weapons, and foreign fighters from nearby Syria.

A leader of Sunni tribes in Mosul opposed to Al Qaeda, Sheik Fawaz Jarba, complained that the sweep was “unorganized” and that public warnings of the coming raids enabled Al Qaeda fighters to flee, as they have done ahead of previous campaigns elsewhere.

“Al Qaeda has gone into hiding and have gone elsewhere,” Mr. Jarba said, adding that his tribal fighters were prepared to join the crackdown but that Mr. Maliki had not asked them to.

American Marines were operating farther south, near Lake Tharthar, a remote desert region that has been a refuge for Al Qaeda fighters and a back channel for supplying the network in the north.

“We’re trying to shut down the rat lines,” Marine Brigadier General Richard Mills, who is heading up the operation, told a briefing at a mobile command post set up in the Mameluke desert.

U.S. Marines yesterday searched an abandoned mud house, uncovering six weapons caches including material for building roadside bombs.Marine Captain Josh Biggers said they discovered evidence that insurgents had recently used the area: broken egg shells scattered across a floor in one room, new electrical fixtures, and the outline on the floor of what troops believe may have been a generator.

“Somebody was definitely here,” Captain Biggers, of Edmond, Okla., said.

Lake Tharthar — once Saddam Hussein’s favorite fishing spot — lies between Mosul and the former Sunni insurgent strongholds of Fallujah and Ramadi. Many Al Qaeda fighters hid in the desolate region after losing control of those cities, and the American military believes the group has been using it for training and as a supply route.