As experts from Iran and the P5+1 met in Vienna to prepare the next round of high-level nuclear talks in Geneva, reports Thursday suggested that Iranian Supreme Leader Ayatollah Ali Khamenei’s health may have taken a turn for the worse.

The 74-year-old Khamenei, who has not been seen in public since giving an address in Tehran three weeks ago, was said by Iranian sources to be suffering from increasingly acute health problems.

Citing Iranian sources, The Times of London reported Thursday that Khamenei, who has served as Supreme Leader since 1989 and who has yet to appoint an heir, may have suffered a relapse of a chronic illness and that he was convalescing after collapsing during a private meeting.

The Times quoted Iranian journalist Hossein Rostami, who runs the website Amariyoun, as saying that “it is not good news coming our way from our master. He urged Khamenei loyalists to “pray deeply for him.”

Rostami was reportedly asked by Iranian authorities not to discuss Khamenei’s health online. On Wednesday, however, he shared Khamenei’s photo on Facebook, along with a poem on health by renowned Persian poet Hafez.

The religious leader reportedly failed to send out his customary greeting to Iranian pilgrims departing for Mecca two weeks ago and didn’t give an address on the central Shi’ite holiday of Ghadeer last week, rousing concern that he was gravely ill.

The festival of Ghadeer marks the appointment of Ali ibn Abi Talib, who is revered by all Shi’ite Muslims as the first imam, and was regarded by the Prophet Muhammad as his successor. The holiday is not recognized by Sunni Muslims, who believe Muhammad left no heir.