Thailand's King Bhumibol Adulyadej, the world's longest reigning monarch, is in an unstable condition after receiving dialysis treatment, the palace has said in a statement.

The Royal Household Bureau said on Sunday that the 88-year-old was given some medicine and put on a ventilator to bring his blood pressure back to normal.

"The medical team are watching his symptoms and giving treatments carefully because the overall symptoms of his sickness are still not stable," the statement said.

Historically the palace has tightly controlled news about his health and it is difficult to ascertain his overall condition. 

In recent months the palace has begun issuing more regular updates that point to a string of major health issues including renal failure.

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Over the past two years he has also been treated for bacterial infections, breathing difficulties, heart problems and hydrocephalus - a build-up of cerebrospinal fluid often referred to as "water on the brain".

He has not been seen in public since January 11 when he spent several hours visiting his palace in Bangkok.

News about the king's health is closely monitored in Thailand, where King Bhumibol is deeply revered. 

Anxiety over the king's health and an eventual succession has formed the backdrop to more than a decade of bitter political divide in Thailand that has included two military coups and often-violent street demonstrations.

Source: News Agencies