Thailand's revered King Bhumibol Adulyadej has been treated for "water on the brain" and a chest infection, the palace revealed in a rare statement, amid public concern over the health of the world's longest serving monarch.

The 87-year-old king, who is perceived as a near-deity by many Thais, has been in Bangkok's Siriraj Hospital since being re-admitted in May, but information on his condition has been scarce.

Doctors at the hospital have "reduced the level of water on his brain", the Royal Household Bureau said in a statement on Monday.

"During his stay at the Siriraj, he has had fever periodically and a raised heartbeat -- assumed to be caused from an earlier lung infection," it said, adding his heart rate had returned to normal after treatment.

Fears over Thailand's future among competing elites as Bhumibol's reign enters its twilight years are seen as a motivating factor behind a decade of political turmoil in the kingdom.

Since 2006, the nation has witnessed two coups, the removal of three prime ministers by the courts and several rounds of street protests that have often ended in violence.

Royalist generals seized power in a coup in May last year after weeks of protests against the civilian government of Yingluck Shinawatra, whose family and their proxies have won every election since 2001.

The Shinawatras are loathed by the royalist elite who accuse them of widespread corruption and subverting the kingdom's political status quo.

Thailand's generals have said they will hand back power once the constitution has been rewritten and corruption expunged.

But critics say the military has used its self-designated status as the defender of the monarchy as a pretext to grab power and ensure the Shinawatras never return to politics.

The king's health is a controversial topic. The Thai monarchy is shielded by one of the world's toughest lese majeste laws and prosecutions have increased dramatically since the military took over.

A man was jailed for 30 years on Friday for "insulting" the monarchy on Facebook, in one of the toughest known sentences for royal defamation. The same day a woman received a 28-year jail term for the offence.

Media have to routinely self-censor when reporting on the monarchy for fear of falling foul of the broadly worded law, which carries up to 15 years in jail for each count of insulting the king, queen, heir or regent.

In recent years, Thailand's 63-year-old Crown Prince Maha Vajiralongkorn has spent much of his time away from the public eye.

But he has stepped in at some official ceremonies in parallel with his father's declining health.

He is due to make a rare high-profile public appearance on Sunday, when he will lead a nationwide bicycling event in honour of his mother -- Queen Sirikit -- that is expected to draw thousands to the streets.