BANGKOK (AFP) - Thailand's revered but ailing King Bhumibol Adulyadej has returned to hospital in Bangkok for medical tests, the palace said on Sunday, amid deep public concern over his health in the politically turbulent country.

The 87-year-old king, the world's longest-serving monarch, is treated as a near-deity by many Thais.

King Bhumibol spent nearly seven months convalescing in Bangkok's Siriraj Hospital after an operation to remove his gall bladder in October.

During his time there he made few public appearances and has not spoken publicly since the operation.

Fears for his health lessened in May when he attended the 60th anniversary celebrations of his official coronation at a palace in Bangkok.

A few days later he was discharged from hospital alongside his also ailing wife Queen Sirikit and the pair travelled to a royal palace in the seaside resort of Hua Hin.

His return to hospital on Sunday will do little to ease Thai fears for the future as the king's reign enters its twilight years.

In a statement released late Sunday the Royal Household Bureau said King Bhumibol would undergo scans to ascertain whether doctors needed to drain fluid from his spinal chord.

Other tests had showed that "his majesty's temperature, blood pressure, heart and respiratory system were normal," the statement read.

The king was accompanied by his wife but the statement did not give any further details on her health.

Most Thais have only ever known King Bhumibol on the throne, and anxiety over the future once his six-decade reign ends is seen as an aggravating factor in Thailand's bitter political schism.

In May 2014, the military took over in a coup following months of street protests that led to the toppling of former premier Yingluck Shinawatra's elected government.

It was the latest chapter in Thailand's long-drawn political conflict, which broadly pits a Bangkok-based middle class and royalist elite - backed by parts of the military and judiciary - against rural and working-class voters loyal to Yingluck and her elder brother Thaksin, also deposed in a coup in 2006.

Thailand's generals have said they will hand back power once the country's constitution has been rewritten and corruption has been expunged.

But critics say the military has used its self-designated status as the defender of the monarchy as a pretext to grab power and ensure the Shinawatras - whose populist parties have won every election since 2001 - never return to politics.

The king's health is a controversial topic. The Thai monarchy is shielded by one of the world's toughest lese majeste laws and prosecutions using the legislation have increased dramatically since the military took over.

Media have to routinely self-censor when reporting on the monarchy for fear of falling foul of the broadly-worded royal defamation law.

Anyone convicted of insulting, defaming or threatening the monarchy can be jailed for up to 15 years on each count.