Prime Minister Ariel Sharon had surgery overnight to replace his breathing tube, the hospital where he is being treated said in a statement Wednesday.

Sharon, who has been in a coma since suffering a stroke Jan. 4, remains in critical but stable condition, the statement from Jerusalem's Hadassah Hospital said.

"The breathing tube was changed overnight due to a technical problem. The successful procedure was conducted in the operating room. The prime minister was taken back to his room in the morning," the statement said.

The 77-year-old Israeli leader, who has been on a respirator, underwent a tracheotomy Sunday to help him breathe better. He has been taken off sedatives.

Outside experts have said his failure to regain consciousness in recent days bodes poorly for his recovery.

After the stroke, Sharon underwent three operations to stop the bleeding in his brain. Medical officials said last week he showed some movement on both sides of his body in response to pain stimuli, but he has shown no signs of improvement since then.

Sharon briefly opened his eyes Monday in response to a recording of his grandson's voice, relatives told doctors, but hospital officials warned the movement may have been an involuntary twitch, and there were no signs that the comatose Israeli leader was any closer to regaining consciousness.