U.S. aircraft pounded Iraqi positions near the northern town of Kalak on Monday, aiding Kurdish fighters as they seized territory from Saddam Hussein's fleeing troops.

Iraqi positions on a ridge west of the Zab river came under relentless attack, sending troops running for cover or speeding away in cars. No anti-aircraft fire was heard.

The aerial bombardment rattled windows in Kalak, but people in town went about their business, shopping in the bazaar and working in the fields. Some sipped tea as they watched the air strikes from their roofs.

Far fewer Iraqi troops were visible than in recent days, which could indicate that government forces were pulling back toward Mosul, the largest city in northern Iraq.

The airstrikes Monday help prepare the way for an advance by coalition troops and U.S.-backed Kurdish militia fighters.The United States sent more than 1,200 paratroopers into northern Iraq last week and has begun coordinating military activities with the Kurds, who have controlled an area known as the northern no-fly zone since the 1991 Gulf War.

Several thousand more U.S. soldiers are expected soon, Hoshiar Zebari of the governing Kurdistan Democratic Party said Monday, declaring that a northern offensive could be a "breakthrough" in the campaign to topple Saddam.

Some limited U.S. ground operations have already begun, Zebari said.

"These are behind enemy lines and many, many operations are very sensitive," he told reporters in Irbil, administrative capital of the Kurdish region.

Over the weekend, Kurdish forces advanced southward toward Kirkuk, Iraq's second most important oil center, and Mosul. Both cities have come under heavy bombardment from U.S. forces.

The Kurds made substantial progress Sunday, taking 10 miles of new territory, but were slowed by dense minefields, said Ares Abdullah, a Kurdish commander.

Elsewhere in the north, Kurdish guerrillas working with U.S. special forces attacked extremists belonging to Ansar al-Islam, a militant group allegedly linked to Osama bin Laden's al-Qaida network.

The attack Friday left 120 to 150 militants dead and dealt "a very serious blow" to terrorism, said Barham Salih, prime minister of the Sulaymaniyah-based Kurdish government. He said 17 Kurds were killed.

"It was a very tough battle," Salih said. "You're talking about a bunch of terrorists who are very well-trained and well-equipped."

American forces were searching a terrorist compound in northeastern Iraq belonging to Ansar al-Islam that was probably the site where militants made a biological toxin, ricin, traces of which were later found by police in London, the Pentagon's top general said Sunday.

U.S. and British forces now control the compound, said Gen. Richard Myers, chairman of the Joint Chiefs of Staff, describing it as a site "where Ansar al-Islam and Al Qaeda had been working on poisons."

Kurdish fighters spent Sunday clearing mines and poking through abandoned Iraqi posts. They carried away war souvenirs and anything with possible value: electrical cables, helmets, vintage gas masks, casings from anti-aircraft artillery.

The Iraqi outposts seemed little more than rough camps. Small cinderblock and mud shelters dotted a clearing — probably a muddy quagmire in rain and a dust bowl in the heat. Roofs were apparently tarps, removed in the withdrawal. Dozens of positions were dug out for tanks or other vehicles.

A team of Kurdish sappers cleared more than 230 anti-personnel mines and 77 anti-tank mines in one day, according to team leader Abdullah Hamza Salim. The light olive anti-tank mines are as big as a layer cake. The smaller mines are black and the size of an ashtray.

The team worked with no protective gear and used sticks to pry up the mines. At least two sappers have been injured since Saturday. Salim said they had received some mine-clearance training but wondered why U.S. experts have not offered help.

"We would welcome the Americans, but they do not come," he said. "We face this danger alone."