Saudi Arabia's de facto ruler on Tuesday offered reassuring words about the health of the Gulf kingdom's ageing monarch who has a lung infection, state media said.

Crown Prince Mohammed bin Salman, first in line to the throne, "reassures everyone about the health of the Custodian of the Two Holy Mosques", the official Saudi Press Agency said on social media, referring to the prince's father, King Salman.

Prince Mohammed also voiced "gratitude to everyone who inquired about King Salman's health" and prayed for his "speedy recovery".

His comments came at the weekly cabinet meeting that King Salman usually attends, though he was absent on Tuesday.

On Sunday the Royal Court said the king had a lung infection and was undergoing a treatment programme involving antibiotics.

A statement earlier that day said he was suffering from a high temperature and joint pain.

Prince Mohammed cancelled a planned trip to Japan "due to the health condition of King Salman", Japan's top government spokesman said on Monday.

King Salman has been on the throne since 2015, although 38-year-old Prince Mohammed was named crown prince in 2017 and acts as day-to-day ruler.

Before that, his most recent hospitalisation had been in May 2022, when he was admitted for a colonoscopy and stayed for just over a week.