BANGKOK (REUTERS) - Thailand's 88-year-old King Bhumibol Adulyadej is under close watch for "irregular contraction of the heart muscles", the Royal Household Bureau said in a statement on Wednesday.

The world's longest-reigning monarch has been treated for various ailments during a year-long hospitalisation in the Thai capital. He was last seen in public on Jan 11, when he spent several hours visiting his Bangkok palace.

Most Thais have known no other monarch, as the King ascended the throne 70 years ago.

He has spent most of the past six years in hospital, and anxiety over his health and the succession has formed the backdrop to decades of political instability in Thailand, where the military took power in a coup two years ago.

The palace said examination last month showed "His Majesty had intermittent stomach pain" and had not been eating sufficiently. Doctors have administered food to the King intravenously, the palace said.

A heart scan to the King showed "irregular contraction of the heart muscles" and he was being administered antibiotics and oxygen, the palace said.

A palace statement on May 20 said an examination of the King showed he had "water on the brain", or hydrocephalus, a build-up of the cerebrospinal fluid that surrounds the brain. He was treated for the same symptom last August.

News about the royals is tightly controlled in Thailand, where laws protecting the royal family from insult make it a crime to defame, insult or threaten the king, queen, heir to the throne or regent.

The King's wife, Queen Sirikit, 83, is also ailing. In a rare statement on her health last month, the palace said Queen Sirikit had "insufficient blood in the brain".