Saudi Arabia's 84-year-old ruler King Salman has been admitted to hospital in the capital Riyadh for gall bladder inflammation, the royal court said on Monday.

It is rare for Saudi Arabia to report on the health of the ageing monarch, who has ruled the top oil exporter and the Arab world's biggest economy since 2015.

The king was "admitted today to King Faisal specialist hospital in Riyadh for some medical tests due to cholecystitis," inflammation of the gall bladder, the royal court said in a statement released by the official Saudi Press Agency.

It did not disclose any further details. Iraqi Prime Minister Mustafa al-Kadhimi's scheduled visit to Saudi Arabia, which was due to start on Monday, has been postponed after the king's hospitalisation, according to the kingdom's foreign minister.

"In recognition of the extremely important visit and a desire to make it a success, our wise leadership in coordination with our brothers in Iraq have decided to postpone the visit," Foreign Minister Prince Faisal bin Farhan wrote on Twitter.

Under the king's rule, Saudi Arabia launched ambitious economic reforms for a post-oil era and gave more rights to women, but also adopted a more assertive foreign policy and entered a war in neighbouring Yemen.

King Salman took the throne after the death of his half-brother Abdullah, who was around 90 years old.  In 2017, Saudi Arabia denied media reports that the king was planning to abdicate in favour of his son, Crown Prince Mohammad bin Salman, who is widely seen as the de facto ruler.

Prince Mohammed's meteoric rise to power has coincided with a sweeping crackdown on critics and dissenters, as well as royal family members.