The health of Thailand’s king, the world’s longest reigning monarch, is “not stable” palace officials have said, as he receives treatment for ongoing medical conditions.

King Bhumibol Adulyadej is widely revered and seen as an arbiter in Thailand’s divided political arena.

He has not been seen in public for months and has spent most of the last year in hospital.

The king has received treatment for a variety of ailments in recent years.

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The palace statement, released late on Sunday night, said the king’s blood pressure dropped as he was being prepared for haemodialysis, a procedure used to treat kidney failure.

He was placed on a ventilator and medication helped return his pressure to normal levels, it added. Doctors are now monitoring his condition closely.

Thailand’s Royal Household Bureau has issued more frequent health bulletins in recent months. Earlier this month, a statement said the king was recovering from a respiratory infection.

The monarch’s health is closely watched in Thailand where he is seen as a unifying symbol amid increased political divisions and violence.