Israeli public radio quoted medical sources as saying on Friday: "The situation is worrying, very worrying, even frightening. The moment of truth is approaching."
The Maariv newspaper's website cited unnamed medical sources as saying early optimism about Sharon's chances for recovery had turned to increasing concern about his condition and they were reviewing his treatment.
Sharon, 77, remained critical, and doctors were conducting a
new battery of neurological tests on Friday.
Doctors have been reducing his sedation to try to bring him out of an induced coma and assess brain damage from a stroke on 4 January.
The hospital in Jerusalem where Sharon is lying in intensive care earlier pronounced Sharon "serious but stable" as he spent his ninth day in a medically-induced coma since suffering a massive brain haemorrhage.
"If by then [when anaesthetics are stopped], there is no sign of improvement, it would need a miracle for Sharon to wake up from his deep coma and be able to function in a way close to normal," Israeli radio said.
According to tests carried out by doctors, the more time passes without Sharon returning to consciousness, with no sign of waking up, opening his eyes or being able to recognise his surroundings, the chances of a man of his age "functioning normally again are very weak", the radio concluded.
Sharon has gone through three bouts of surgery to stem bleeding in his brain since being admitted to hospital after the stroke.