Long-time chief Palestinian negotiator Saeb Erekat was in "critical" condition and in a medically induced coma on Monday, said the Israeli hospital in Jerusalem treating him for coronavirus complications.
The Palestine Liberation Organization said on Sunday that Erekat had been admitted to Hadassah Ein Kerem hospital's coronavirus intensive care unit due to "chronic health problems in the respiratory system".
The 65-year-old had tested positive for Covid-19 on October 19, the PLO said.
Erekat had "arrived in serious condition" and was receiving oxygen, the hospital said on Sunday, adding that his condition was "serious" but "stable".
On Monday morning, Hadassah said that after a "quiet night," his condition "deteriorated and is now defined critical and due to respiratory distress, he is anaesthetised and ventilated."
Erekat, who lives in the biblical town of Jericho in the Israeli-occupied West Bank, underwent lung-transplant surgery in the United States in 2017.
"Mr Erekat is a challenge to treat for coronavirus since he had lung transplants, he is immunosuppressed, and has another bacterial infection in addition to coronavirus," Hadassah said on Monday.
Hadassah said it was "is in contact with international medical authorities regarding the treatment policy of this complex patient".
The Jerusalem-born Erekat serves as the PLO's secretary-general and in remains Palestinian president Mahmud Abbas's inner circle.
He has been a key figure in Palestinian politics for decades, often serving as a main interlocutor for foreign envoys and the international media.
He has consistently voiced support for a two-state solution to the Israeli-Palestinian conflict.
There have been 42,490 confirmed cases of the novel coronavirus in the West Bank, including 381 deaths.