"Currently, we have about 20 who are suspected of being from al Qaeda," Lt. Gen. Ricardo Sanchez told reporters. "We are still questioning them" and have not determined absolutely whether they are members of Osama bin Laden's terrorist organization.
U.S. officials have said they suspect foreign volunteers, including some from al Qaeda, have slipped across the borders into Iraq to take part in a "holy war" against the U.S.-led occupation.
However, a number of U.S. commanders have said they were uncertain about the numbers of foreign fighters and their role in the insurgency.
Asked about foreign fighters, Sanchez said "hundreds" of foreigners cross the border area to carry out attacks here. Sanchez was asked how close U.S. forces had been to capturing Saddam Hussein, Sanchez replied only: "Not close enough."
American commanders have speculated that they are facing attacks from Saddam supporters, religious extremists and foreign fighters. U.S. officials have said at least some of the attacks may have been orchestrated by Saddam's former deputy Izzat Ibrahim al-Douri, who may have forged an alliance with the Kurdish religious extremist group Ansar al-Islam.
Ansar al-Islam is believed to have ties to al Qaeda. It was unclear whether Sanchez was referring to Ansar fighters when he said the Americans were holding about 20 al Qaeda suspects.
Sanchez said there was "no doubt" that the insurgency had intensified and that the guerrillas "have developed their capability" employing rockets and mortars in recent attacks.
Sanchez said most of the foreign fighters enter the country from Syria and across the northeastern border with Iran. He said Iraqi immigration authorities were now monitoring the border more effectively to make sure no one enters the country with forged documents.
In other developments:
In Basra Tuesday, an explosion destroyed two cars on a road frequently used by British troops. Soldiers immediately blocked off access to the site, but Iraqi police said four civilians were killed and three injured in the blast.
The commander of U.S. forces in Iraq, Lt. Gen. Ricardo Sanchez, told reporters Tuesday that although attacks against his troops have increased, the insurgents know "that from a military point of view, they can't defeat us."
Sanchez also said that Iraqi police apprehended an ambulance Monday full of explosives, the second time that such a vehicle has been seized in the past two weeks.
On Monday, U.S. jets dropped three 500-pound bombs in the Fallujah area after three paratroopers from the 82nd Airborne Division were wounded in an ambush. There was no report of casualties from the bombing.
"Neither America, nor the father of America, scares us," said one resident, Najih Latif Abbas. "Iraqi men are striking at Americans and they retaliate by terrifying our children."
In Mosul, an oil official was wounded and his son killed when assailants opened fire at their car in the northern city Monday, his family said.
Mohammed Ahmed Zibari, the Northern Oil Company's distribution manager, was headed to work when gunmen riddled his car, his brother Nawzat Zibari said. The brother speculated that Zibari was killed by "terrorists" because they believed he was cooperating with the Americans.