Saudi Arabia's mass purge of elite figures accused of corruption "raises a few concerns", US Secretary of State Rex Tillerson said yesterday, as Washington's Gulf ally probes graft allegedly worth $ 100 billion.

The unprecedented roundup has seen more than 200 princes, ministers and businessmen detained in what has widely been seen as a move by Crown Prince Mohammed bin Salman to consolidate his power ahead of his accession to the throne.

The domestic upheaval has come as Riyadh is locked in an intensifying proxy war with regional rival Tehran that has triggered a crippling aid blockade

of neighbouring Yemen which threatens what the United Nations has warned could be the world's worst famine in decades.

Tillerson, who is accompanying President Donald Trump on an Asian tour, said he believed the mass arrests ordered by a new anti-corruption commission headed by

Prince Mohammed were "well intended".

But he cautioned that the lightning roundup "raises a few concerns until we see more clearly how these particular individuals are dealt with".

The top US diplomat's comments marked a step back from the fulsome support offered earlier this week by President Donald Trump, who said he had "great confidence" in what the crown prince and his father King Salman were doing.

"Some of those they are harshly treating have been 'milking' their country for years!" Trump had tweeted.

The Saudi authorities have frozen the bank accounts of those accused and warned that assets related to the alleged corruption

cases will be seized as state property.

High-profile figures, including billionaire tycoon Prince Al-Waleed bin Talal, are among those detained.

With the purge, which analysts describe as a bold but risky power play, the crown prince has centralised power to a degree that is unprecedented in recent Saudi history.