Saudi Arabia's ailing King Abdullah bin Abdulaziz, 91, has been admitted to hospital for "medical checks," the royal court has said.

The king, whose age and frequent hospitalisation have raised concerns about the future leadership of the key oil producing country, was admitted to the King Abdulaziz Medical City in Riyadh, a court statement carried by the official SPA news agency said on Wednesday.

Abdullah's half brother Salman, 77, is next in line to the throne. He was named crown prince in June 2012 following the death of Prince Nayef bin Abdul Aziz.

Salman has been representing the king at most public events, including the latest Gulf summit in Qatar this month, due to the monarch's ailing health.

The king's absence from the public gaze for some time last year prompted rumours on social media networks that his health was deteriorating.

He underwent two operations in October 2011 and November 2012 to correct "ligament slackening" in the upper back.