Saudi Arabia's former foreign minister Saud al-Faisal has died aged 75, just two months after he lost the job owing to health issues.

Prince Saud was the world's longest-serving foreign minister, holding the position for 40 years.

Riyadh has not said what caused the veteran diplomat's death but an official announcement is expected shortly. Saud, a high-profile diplomat in the kingdom's ruling elite, has had multiple surgeries in the recent past.

"I wish I could deny the rumour of the news of your death. The eye tears, the heart saddens," said Saudi Arabia's Foreign Ministry spokesperson Osama Nugali.

Saud's nephew Saud Mohammed al-Abdullah al-Faisal wrote on Twitter: "May God accept him in paradise."

Saud, born in 1940, is credited with calibrating Saudi Arabia's diplomatic clout not just in the Middle East but also across the world. Steering Saudi Arabia's ties with the Western world, he was also pivotal in the kingdom's emergence as a powerhouse in the region.

Saud was appointed foreign minister in 1975 and held that position until being replaced by Adel al-Jubeir in April 2015, chiefly due to health reasons.

Former British Prime Minister Tony Blair said in a statement: "Prince Saud al-Faisal was a man of great humility, compassion and wisdom. He worked tirelessly for peace."

"Saud was not only the longest-serving foreign minister, but was also one of the wisest," US Secretary of State John Kerry said, according to a tweet by the US embassy.

President Barack Obama has also offered his condolences.