Khamenei “had surgery some time last week for bowel obstruction after suffering extreme stomach pains and high fever,” the NYT reported, citing one of the people.

Khamenei is currently being monitored “around the clock” by a team of doctors after undergoing the surgery, according to the report.

Khamenei’s condition was considered “critical last week, but has improved, and he is currently resting,” the report added. “His doctors are monitoring him around the clock and remain concerned that he is still too weak to even sit up in bed.”

Khamenei’s office canceled all meetings last week, including an annual meeting with the Assembly of Experts on September 6 because he was “too ill to sit up,” the report said.

The assembly is a clerical body that supervises, appoints and in theory, can sack the Supreme Leader.

Khamenei, 83, has been the supreme leader of Iran since 1989. He is the country’s highest authority and has the final say on all state matters.