Washington: US President Donald Trump on Monday endorsed a move by Saudi Crown Prince Mohammed bin Salman bin Abdulaziz Al Saud over the arrests of princes, ministers and investors in an anti-corruption purge.

The endorsement cemented a US-Saudi relationship that has improved dramatically under Trump's presidency, partly because of both leaders' vision of confronting Iran.

Trump tweeted on Monday that he had "great confidence in King Salman and the Crown Prince of Saudi Arabia" following the mass arrests.

Trump also tweeted that "they know exactly what they are doing," adding: "Some of those have been 'milking' their country for years."

Trump's son-in-law, Jared Kushner, the president's senior adviser, recently returned from Saudi Arabia.

The White House said at the time the trip was reported that it was within the context of Kushner's efforts on Israeli-Palestinian peace conversations.

Among those arrested were billionaire investor Prince Alwaleed bin Talal.

Also detained was Prince Miteb bin Abdullah bin Abdulaziz Al Saud, who was replaced as minister of the National Guard.

US-Saudi ties had been strained under Barack Obama over the negotiation of the Iran nuclear deal.

But the Trump administration has vowed to confront Iran much more aggressively in the region, where it shares the view that Tehran is fomenting instability via a number of proxies in Lebanon, Syria and Yemen among other countries.

Saudi Arabia accused Lebanon on Monday of declaring war against it because of what it called aggression by the Iran-backed Lebanese group Hizbollah.

Lebanese politician Saad Hariri quit as prime minister on Saturday, announcing his resignation from Riyadh and blaming Iran and Hizbollah in his resignation speech.

Also on Saturday, Saudi Arabia’s air defence forces intercepted a ballistic missile fired from Yemen over the capital, Riyadh.

The Pentagon praised Saudi Arabia for "exposing" Iran's role in Yemen and Tehran's provision of missile systems to Houthi militia fighting there.