WASHINGTON (AFP) - The United States on Wednesday said it would continue its "close, productive relationship" with Saudi Arabia amid a radical shake-up of King Salman's inner circle.

Acting State Department spokeswoman Marie Harf also paid tribute to outgoing Saudi foreign minister, Prince Saud al-Faisal, who is stepping down after holding the post for four decades.

"The alliance, as we all know, between Saudi Arabia and the US is historic and enduring, and we will continue to maintain our close, productive relationship as we work together to address a number of serious challenges," Harf said.

Secretary of State John Kerry and "the rest of our team honour the service of the longest-serving foreign minister of any country, certainly a friend of the secretary," she told reporters.

The United States would remember Saud, who was stepping down for health reasons, "as a skilled diplomat, wise counselor on regional issues, and a good friend to the United States."

Harf added the United States looked forward to working closely with his successor, Saudi Arabia's longtime envoy to the United States, Adel al-Jubeir, who met with Kerry on Wednesday at the State Department to discuss a range of issues.

A vital regional player, Saudi Arabia is a key US ally in a host of burning concerns, including the battle to drive Islamic militants out of Iraq and Syria, and the upheaval in Yemen where Riyadh has been leading air strikes to halt the march of Shi'ite Huthi militias.

But the oil-rich Gulf kingdom was been wary of the US rapprochement with its main regional foe, Iran, and Kerry has worked hard to keep Saudi leaders briefed about the nuclear negotiations.