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Thailand's 88-year-old king, the world's longest-reigning monarch, has undergone heart surgery, the Thai Royal Household Bureau said.

King Bhumibol Adulyadej has had a lengthy spell in hospital in Bangkok and has not been seen in public for months.

The king is widely revered and seen as an arbiter in the country's divided political arena.

Thailand is preparing to celebrate 70 years of his reign on Thursday.

The monarch was treated for narrowing of the arteries with "satisfactory results", the palace said in a statement.

Doctors performed a procedure known as balloon surgery to widen the arteries on Tuesday after tests had shown insufficient blood in the heart muscles, the statement said.

In the past month, the king has also been treated for a build up of fluid surrounding the brain and a swollen lung.

An X-ray on Saturday showed less fluid around the brain, the statement said.

The monarch's health is closely watched in Thailand as he is seen as a unifying symbol and pillar of stability in a society that has suffered increased political divisions and violence.