Oman says Sultan Qaboos is in a stable condition

Ruler, 79, returned recently from medical check-up in Belgium

Sultan Qaboos was the region's longest serving monarch. AP

Sultan Qaboos is in a stable condition after recently travelling to Belgium for a medical check-up, Oman's royal court said on Tuesday.

"The Sultan is in a stable condition and following a scheduled treatment programme," the official ONA news agency quoted the royal court as saying.

“The Sultan thanks his people all around the world for sharing their good wishes and prayers for him.”

The court announced on December 7 that Sultan Qaboos, 79, was travelling to Belgium for "a limited period of time", but did not give details on his condition.

He returned to Oman but rumours soon spread that he faced a relapse.

Sultan Qaboos, who has ruled Oman since 1970, has rarely appeared in public since eight months of treatment in Germany in 2014 and 2015.

His ill health and repeated hospital spells in Germany have focused attention on the fact that he has no designated successor. He is unmarried and has no children or brothers.

Under the constitution, Sultan Qaboos must write a letter designating a successor from the ruling dynasty, to be opened in the event that his family cannot agree on his replacement within three days of his death.