Muscat (AFP) - Sultan Qaboos who has ruled Oman for almost half a century is in stable condition after recently travelling to Belgium for a medical checkup, his royal court announced Tuesday.

"The sultan is in a stable condition and following a scheduled treatment programme," the official ONA news agency cited the royal court as saying, without giving details.

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

Qaboos, the longest ruling Arab monarch still in office, is believed to be suffering from colon cancer and has rarely appeared in public since undergoing lengthy treatment in Germany in March 2015.

The sultan's ill health and repeated hospital spells in Germany have focused attention on the fact that he has no designated successor.

Qaboos, who has ruled the Gulf sultanate since 1970, is unmarried and has no children or brothers.

Under the constitution, the sultan 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.