Thailand's King Bhumibol Adulyadej, the world's longest reigning monarch, is in an "unstable condition" and has been placed on a ventilator, according to a rare palace bulletin about his ailing health.

The elderly monarch had undergone a procedure to purify his blood and drain fluid from his spine which lasted two and a half hours, palace officials said.

King Bhumibol is hugely popular in Thailand, though he has not appeared in public for nearly a year due to ill health.