U.S. warplanes and artillery units fired on the insurgent-held city of Fallujah (search) overnight, killing four people and wounding six, hospital officials said Sunday.

An artillery barrage on an industrial area in Fallujah early Sunday left two people dead and two others wounded, said Dr. Ahmed Khalil of the Fallujah General Hospital (search).

Late Saturday, U.S. warplanes unleashed missiles on a main street in the city center, killing two people and wounding four, said another hospital official, Dr. Rafea Heyad.

The U.S. military had no immediate comment about the attacks on the industrial zone, but said Saturday's strikes hit a checkpoint manned by insurgents linked to alleged terror mastermind Abu Musab al-Zarqawi (search).

"The illegal checkpoint used blockades to disrupt security, intimidate and harass local citizens and interrogate and detain local civilians," the military said in a statement.

The attacks were the latest in a series of strikes in and around Fallujah since late Thursday.

Al-Zarqawi's Tawhid and Jihad group has claimed responsibility for a series of deadly attacks in Baghdad during the past week as well as a number of car bombings, kidnappings and other assaults meant to destabilize the U.S.-backed interim authorities.

American troops have not entered Fallujah since ending a three-week siege of the city in April that left hundreds dead.