Iraqi insurgents have stepped up attacks on U.S. troops in recent weeks, the commander of American forces said Wednesday, as ambush bombers struck again in this tense Sunni Muslim area west of Baghdad, in the northern city of Mosul and in the heart of the capital.

The U.S. commander, Lt. Gen. Ricardo Sanchez (search), said some of the attacks may be carried out by people with links to Al Qaeda, but he added "we don't have any confirmed Al Qaeda operatives in custody at this point."

Elsewhere, U.S. troops of the 4th Infantry Division arrested more than a dozen suspects, including a former major general, in pre-dawn raids Wednesday north of Baghdad.

The Baghdad bombing, which occurred as a convoy passed through a tunnel in the center of the capital, slightly injured two U.S. soldiers, who were returned to duty after treatment, a U.S. officer at the scene reported.

In Fallujah, witnesses said four Americans were carried away on stretchers after a roadside bomb exploded beside a three-vehicle convoy. The U.S. military in Baghdad had no report on the incident.

After the attack, residents cheered and swarmed over one disabled vehicle, looting its contents and setting it afire. It was the third attack against American troops in the Fallujah area in as many days.

Elsewhere, one soldier from the 101st Airborne Division was slightly injured when a bomb exploded in front of his convoy in Mosul, 225 miles north of Baghdad, the military said.

Sanchez, the American commander, acknowledged that attacks against his troops have increased in the last three weeks, especially in Anbar province which includes Fallujah.

"The number of wounded and the number of engagements in last three weeks have been a little bit higher than we've seen before," Sanchez said. "We've had an average number of engagements from 20 to 25 (daily). We've seen a spike up to 35 in last three weeks."

Sanchez said he believed operates of the extremist Ansar al-Islam, a group linked to Al Qaeda, as well as members of Usama bin Laden's organization are operating inside Iraq.

"We have arrested members of Ansar al-Islam (search)," he said. "We know there are organizations that have links back to Al Qaeda but we don't have any confirmed Al Qaeda operatives in custody at this point. We do have some Al Qaeda-linked personnel in our custody. They continue to operate. We know that they're here. We know where they're operating, what areas they're operating in."

He did not elaborate.

Sanchez also attributed the increase in part to combat activity here in Anbar province, which extends from the Euphrates River valley to the Jordanian, Syrian and Saudi borders. The most populated part of the province is the responsibility of the 82nd Airborne Division.

"Before the 82nd came in, we very clearly stated we were going out to the west," Sanchez said. "We've been in combat force posture out in Anbar province. We knew this was an area that was a challenge for us. And we clearly stated back then there was still a lot of fighting to be done. And there would be an increase in engagements."

The raids by the 4th Infantry Division were centered around Saddam Hussein's hometown of Tikrit and in Baqouba, about 30 miles northeast Baghdad. U.S. officers said an Iraqi major general, who was not identified, was seized in the Baqouba operation.

In Tikrit, 10 suspects, including six people believed linked to the Iraqi resistance, were taken into custody, the military said.

Despite an escalation of military operations, Sanchez said the coalition planned to lift the nighttime curfew in Baghdad during Ramadan (search), the Muslim holy month of fasting which begins this weekend. Muslims must abstain from food, drink and sex during daylight hours, and Ramadan festivities last far into the night.

The continuing attacks on the U.S. occupation army came as Washington prepared for a conference in Madrid on Thursday and Friday to win international aid to rebuild Iraq — help the Americans hope will eventually be accompanied by foreign troop reinforcements.

Sanchez insisted the U.S.-led coalition was making progress in restoring order six months after the collapse of Saddam's rule "but we need to accelerate it and accomplish it across all lines of operation — economic, political, security."

He said that restoring order and getting more Iraqis back to work "will contribute to eliminating some of the anti-coalition forces throughout the country." Iraqi officials estimate unemployment nationwide at more than 70 percent.

In the southern city of Najaf, a small band of gunmen staged a midnight attack on the headquarters of a leading Shiite Muslim political organization, but no casualties were reported, said a spokesman for the group, the Supreme Council for the Islamic Revolution in Iraq.

The spokesman, who identified himself as Abu Ahmed, said four of six attackers were captured and admitted they were loyalists of Saddam's Baath Party. During years of Shiite repression under Saddam, the group fueled opposition to his government from exile. Since Saddam's fall in April, it has taken a prominent role in the political transition in Iraq.