Saudi Arabia's King Salman on Wednesday named his powerful interior minister as heir in a major shakeup that also saw the world's longest-serving foreign minister replaced.

A royal decree removed Crown Prince Moqren bin Abdul Aziz bin Saud as next in line to the throne and replaced him with Prince Mohammed bin Nayef, who headed a crackdown on al-Qaeda in the oil-rich kingdom a decade ago.

"We have decided to respond to his highness and what he had expressed about his desire to be relieved from the position of crown prince," said a statement from the royal court, carried by the official Saudi Press Agency.

It added that Moqren, 69, was also relieved of his position as deputy prime minister in the world's largest oil exporter, but insisted "he will always remain in high regard".

The decree named Prince Mohammed bin Nayef as crown prince as well as deputy prime minister and said he will continue in his position of interior minister and head of the political and security council, a coordinating body.

A separate decree Wednesday said King Salman's son, Prince Mohammed bin Salman, who is in his early 30s, will be deputy crown prince.

He retains his position of defence minister, in which he has recently played a key role in a Saudi-led coalition conducting air strikes on Yemeni rebels.

Jamal Khashoggi, general manager of al-Arab News, has previously described Mohammed bin Nayef as "receptive to ideas of reform."

"But he is good at making everybody think he is in their camp," he said when Prince Mohammed was appointed in 2012. "That's what makes a successful politician."

The shuffle, as many have noted on social media overnight, brings a younger generation to the fore.

A major reshuffle in King Salman government with a clear indication that young faces are taking over. — Dr Jaber Alsiwat (@jsiwat) April 29, 2015

Kristian Coates Ulrichsen, a Baker Institute fellow for the Middle East, described the move as a "significant reshuffling of the old guard and a statement of intent by the King that he intends to accelerate the transition to a new generation of leaders."

The dismissal of Moqren removes one of the few remaining high-level officials from the era of King Abdullah, who died on 23 January and was replaced by Salman, 79.

Moqren would have been the last son of the kingdom's founder, Abdul Aziz bin Saud, to rule.

He was a confidant of the late Abdullah, who appointed him deputy crown prince behind then-crown prince Salman in March 2014, an unprecedented move.

Moqren's removal leaves bin Nayef as the first of the second generation, or grandsons of Abdul Aziz, in line to lead the country.

The appointment of bin Nayef, 55, further solidifies control of Salman's Sudairi branch of the royal family. Their influence had waned under Abdullah.

A Western diplomat said Moqren had really only a "protocol" position under King Salman, describing bin Salman as "the strong man in Saudi Arabia".

New foreign minister

Under Salman, Saudi Arabia has adopted a more assertive foreign policy, leading the Arab-dominated coalition targeting Houthis in neighbouring Yemen since late March.

In another major change, Saudi Arabia's envoy to the US, Adel al-Jubeir, was appointed foreign minister, a royal decree said.

He replaces Prince Saud al-Faisal who "asked to be relieved from his duties due to his health condition," said the decree carried by the SPA.

Prince Saud had held the post since 1975, making him the world's longest-serving foreign minister.

Born in 1940, he was in the US for back surgery when Salman acceded to the throne.

The decree said Prince Saud has been appointed as an adviser and a special envoy of the king, as well as a supervisor on foreign affairs.

But Khashoggi denied his appointment heralded anything more significant.

“Adel is a career diplomat, he will implement the policy that is being created by the seniors in Riyadh and he will be part of the policy making process," he told MEE. "Therefore, I do not anticipate any changes.”

“It's plain what Saudi Arabia is up to and Adel will have to implement the view of this policy," he added.

Jubeir came to attention answering reporters' questions in the US in defence of his country's decision to participate in an aerial campaign in Yemen.

His appointment is a rarity as the position of foreign minister is usually held by a member of the ruling family.

Salman also named a new health minister in Wednesday's reshuffle, the second major government shakeup since he took office.