BAGHDAD, Iraq — U.S. and Iraqi forces captured more than 100 suspected guerrillas in raids across the country, arresting one of Saddam Hussein's intelligence chiefs and another Iraqi believed involved in a suicide bombing last month, a U.S. commander said Thursday.

The raids on Wednesday and Thursday occurred as daily attacks on U.S. forces are climbing after a recent lull. Rebels lobbed a mortar shell Thursday at a checkpoint near Baghdad International Airport, killing one U.S. soldier and wounding another, the U.S. command said.

The attack outside the airport, which serves as a major American military base, brought to 529 the number of American troops killed since the Iraq war began March 20.

American forces are also tracking a shadowy militant group that claimed responsibility for Sunday's back-to-back suicide bombings, said U.S. Army Brig. Gen. Mark Kimmitt. The bombings, which devastated gatherings at Kurdish political offices in the northern city of Irbil, killed at least 109 people, including senior Kurdish politicians who were strong U.S. allies.

A statement from a group calling itself the Ansar al-Sunna Army said it targeted the "dens of the devils" because of the Kurds' ties to the United States.

Kimmitt said the U.S. military believes Ansar al-Sunna is a splinter group of Ansar al-Islam, which is believed to be linked to the al-Qaida terrorist network. The claim could not be confirmed, and the military said investigators from the military and the U.S. Federal Bureau of Investigation had made no progress in finding those responsible for the catastrophic bombings. But a Kurdish politician said its investigators were following clues picked up in the bombings' aftermath.

The Ansar al-Sunna statement may be significant for other reasons, however. Few previous attacks on U.S. forces and Iraqi civilians have been followed by claims by groups claiming to have arranged them. Most are chalked up to a little-understood guerrilla insurgency that bears no name.

"We are certainly going to follow up on the claims," said Kimmitt, the command's deputy chief of staff for operations. "That's the only claim we have right now. We aren't closer than we were 24 hours ago or 48 hours before ... either on the group responsible or on its motivation."

Kimmitt said Ansar al-Sunna — which casts itself as protector of Iraq's Sunni Muslims — also claimed responsibility for Saturday's car bombing outside a police station in the northern city of Mosul that killed nine people.