Houston, Jan 16 (Agencies): US Congresswoman Gabrielle Giffords, who was gravely injured in a gun attack at a public event in Arizona's Tucson area, is breathing on her own as doctors removed her from a ventilator, although she remains in critical condition.

40-year-old Giffords, a Democrat, underwent a procedure on Saturday to have her breathing tube removed and a tracheotomy tube inserted in her windpipe to protect her airways, according to hospital sources.

She is now off ventilator, although she remains in critical condition at the University Medical Centre, they said.

Giffords, a House Representative, had been breathing on her own since she was shot in the head at the harrowing mass shooting on January 8 by a 22-year-old gunman, who killed six people and injured 12 others, but doctors had left the breathing tube in as a precaution.

A feeding tube was also put into provide nutrition. Those procedures are not out of the ordinary for brain-injured patients.

"Her recovery continues as planned," the hospital said in a statement.

Her husband, astronaut Mark Kelly, has remained by her bedside. One of those injured in the attack was discharged on Saturday while two others remained in good condition.