Shimon Peres' condition has deteriorated two weeks after he suffered a major stroke, a person close to the former Israeli president said.
The person did not disclose further details about Mr Peres' worsening condition.
Mr Peres, 93, was taken to hospital two weeks ago after suffering a stroke that led to bleeding in his brain.
He has been sedated and on a respirator during most of his time in hospital.
Mr Peres, a Nobel Peace Prize laureate, is the elder statesman of Israeli politics, one of the country's most admired leaders and the last surviving link to its founding fathers.
Over a seven-decade career, he has held virtually every senior political office, including three stints as prime minister.