Chicago (AFP) - As United Airlines struggles to recover from the public relations disaster sparked when a passenger was violently removed from a flight in Chicago, the airport security officers involved in the incident are facing tough scrutiny.

Officials in the midwestern US city are looking at why the three officers -- all now placed on administrative leave -- were on the plane in the first place, amid criticism from elected officials and the United pilots' union.

The officers -- who are unarmed and meant to back up local law enforcement -- were called by airline employees after the passenger, David Dao, refused to give up his seat for crew that needed to be repositioned for other flights.

The resulting altercation left Dao with a broken nose, concussion and other injuries, according to his lawyers, who have said he is likely to sue.

But Jeff Redding, who is in charge of safety and security at the Chicago Department of Aviation, which operates O'Hare International Airport, says airport security officers are not supposed to respond to such calls.

"If it is a customer service-related incident, then you don't need to board the plane at all," Redding told a group of Chicago city council members on Thursday.

The official however could not immediately say how his officers were instructed about the use of force.

The agency did not immediately respond to AFP's request for comment.

Once the officers were on the plane, they bungled the situation, according to United's pilots' union.

The United Master Executive Council on Thursday accused the security officers of actions that were "grossly inappropriate."

"For reasons unknown to us, instead of trained Chicago Police Department officers being dispatched to the scene, Chicago Department of Aviation personnel responded," the union said.

The airport security officers, while required to meet minimum police standards and go through academy training, are not in fact police, and cannot carry guns or arrest people.

"We are going to thoroughly review every aspect of our operation," said Chicago's aviation chief Ginger Evans.

The Port Authority of New York and New Jersey, responsible for the major airports in the New York area, sent out a new directive Friday noting that their officers would not assist in removing a passenger in an overbooking situation.