WASHINGTON (AP) _ Personnel from a private U.S. security firm in Afghanistan have engaged in ``aggressive behavior'' while protecting President Hamid Karzai, the State Department said Wednesday.

Spokesman Richard Boucher said U.S. concerns about inappropriate activities by some guards from the Texas-based DynCorp firm have been passed on to company officials.

The State Department's bureau of diplomatic security has been responsible for Karzai's protection since 2002. DynCorp personnel have assisted in that task but the State Department provides overall supervision.

``It's a very difficult mission. It's a very dangerous mission,'' Boucher said. Less than a month ago, Karzai was the target of an assassination attempt when a rocket was fired at his helicopter near the city of Gardez. He escaped injury.

Afghan authorities say they have uncovered previous assassination plots against Karzai by an ``international terrorist group.''

Boucher said that overall the security assistance DynCorp provides has been outstanding.

At the same time, he said, ``we have seen these reports and from time to time have heard other reports of aggressive behavior by some of the contractors.

``And we have addressed that issue with the contractors on the ground, as well as with DynCorp management.''

Boucher was responding to a New York Times article which said the Dyncorp guards, at best, reinforce the stereotype of Americans as ``brawny and boorish.'' The article was titled, ``The Intimidating Face of America.''

Efforts to obtain comment from DynCorp officials were unsuccessful. The State Department declined to provide specific examples of inappropriate behavior by DynCorp personnel.