Thailand's 88-year-old king, the world's longest reigning monarch, has shown signs of improvement following treatment for a blood infection, the palace said in a statement on Sunday.

King Bhumibol Adulyadej has been treated for various ailments during a year-long hospitalization in the Thai capital.

The king's fever has subsided and the severity of his blood infection has eased, the palace said.

The statement came two days after the palace released a monthly update on the king's health, which said he had water in his lungs and reduced kidney function.

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 military takeovers and sometimes violent street demonstrations.

According to the Sunday's statement, an x-ray showed the king still had some water in his lungs and a team of physicians had to administer antibiotics and renal therapy.

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