Thailand's 86-year-old king remains hospitalized with an infected colon, a month after he underwent surgery to remove his gallbladder, the palace said Thursday.

King Bhumibol Adulyadej, the world's longest-reigning monarch, was admitted to a Bangkok hospital early last month with an inflammation of his gallbladder, which was removed through laparoscopic surgery.

He began suffering last Tuesday from a bout of diverticulitis, an infection of pouches in the wall of the lower part of the large intestine, the palace said in a statement. After he improved, doctors stopped giving him an antibiotic, but his fever returned and the monarch experienced stomach pain and "faster breathing, faster heart rate and some diarrhea," it said.

It said doctors are giving him a new type of intravenous antibiotic and his fever has declined. His overall condition has improved but doctors will give him the antibiotic for a longer period of time, the palace said.

Worries about the king's health and succession have contributed to Thailand's political instability over the past eight years.

The king spent almost four years in the hospital before leaving in August last year. When he was admitted in 2009, the palace said he was suffering from a lung inflammation, but it has never fully explained his long hospital stay. He suffers from a variety of ailments usually associated with age.

Most Thais hold great affection for Bhumibol, who took the throne in 1946 and is seen as a moral authority who stepped in repeatedly over the years to unify the country despite his lack of a formal political role.

His fading from public life and the palace's perceived role in the nation's latest political battles that started in 2006 have tarnished the institution in recent years, undermining what had previously been near-universal respect for the royal institution.

Over the weekend, the king made an appearance in a wheelchair on the grounds of the hospital, where he paid homage to a statue of his father, a doctor, and sat on the bank of the Chao Phraya river.

The king's health has raised concerns about what will happen after his passing. Crown Prince Vajiralongkorn does not command the same respect and affection as the king, who was actively involved in the country's economic development.

Open discussion of the monarchy is constrained by a strict lese majeste law that makes criticism punishable by up to 15 years in prison. (*****)

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