Saudi crown prince Mohammed bin Salman's aggressive power grab represents a huge gamble on the stability of his kingdom and its neighbors, but Donald Trump is not one to worry.

The Washington foreign policy establishment may be agog at the young leader's "anti-corruption" purge of potential foes within the Saudi elite, but the US government barely flinched.

No one is quite sure whether MBS' bold move will leave him as the uncontested leader of a more modern, open Saudi Arabia -- or open the door to chaos, rebellion or a regional war.

But Trump is bringing the United States along for the ride.

"I have great confidence in King Salman and the Crown Prince of Saudi Arabia, they know exactly what they are doing," Trump tweeted on Monday, after a round of high-profile arrests.

Dozens of prominent Saudis, among them a dozen royals including billionaire tycoon Prince Al-Waleed bin Talal, were arrested over the weekend and may now face trial.

The arrests were the first order by an anti-graft commission headed by Prince Mohammed himself, and many outside observers saw it as a politicized move against rivals within the elite.

Trump, who has tangled publicly with Al-Waleed in the past, had no such doubts, declaring: "Some of those they are harshly treating have been 'milking' their country for years!"

MBS' move comes as he moves to accelerate his Vision 2030 project to modernize the conservative kingdom, but also as Riyadh takes a more aggressive stance in its wider region.

Saudi-led forces are waging a fierce war against Iran-backed Huthi rebels in Yemen and Riyadh has led its allies in a boycott of Gulf emirate Qatar, to growing US frustration.

In the wake of an alleged Huthi missile attack against Riyadh airport, the kingdom has accused Iran of "direct aggression" and threatened an equally direct though unspecified response.

And Riyadh was deeply involved in Lebanese premier Saad Hariri's decision to resign his post and flee to the kingdom, precipitating a new political crisis in Beirut.

Some might see this string of crises as headaches the already tumultuous Middle East could do without, but Washington is still standing four square behind its ally.

"We continue to encourage Saudi authorities to pursue the prosecution of people they believe to have been corrupt officials," State Department spokeswoman Heather Nauert said.

"We expect them to do it in a fair and transparent manner."

- The hand of Iran -

And, pressed on whether Riyadh's aggressive stance towards Iran or its interference in Lebanon was a cause for concern, the spokeswoman instead pointed the finger at Tehran.

"I think we know exactly where the responsibility lies in the region for much of the destabilization," Nauert said.

"We've seen the activities of Iran in Yemen. We've seen the hand of Iran in Syria. We've seen the hand of Iran elsewhere."

Whereas former US president Barack Obama sought to "rebalance" US interests in the Middle East, hoping to straddle the Iran-Saudi divide, Trump has very publicly joined one camp.

His first foreign trip took in Riyadh and his son-in-law Jared Kushner has reportedly formed a close bond with 32-year-old MBS.

But, despite Obama's caution, past US presidents have always maintained close ties with an oil-rich kingdom in the name of regional stability. Is this now a mistake? Perhaps.

"I am very concerned about Trump's unconditional support for MBS," says Barbara Slavin, a journalist and Middle East scholar at the Washington policy institute The Atlantic Council.

For Slavin the feud with Qatar, war in Yemen and domestic crackdown are "extremely risky policies that may well backfire."

Warning of the risks of a backlash at home and war abroad, she told AFP: "In such a situation, it is better to have options, not to put all your diplomatic eggs in one basket."

- 'Risky move' -

And even observers with more sympathy for MBS' objectives, warn that the weeks and months to come will contain many risks for the young heir and his White House friends.

Lori Plotkin Boghardt, an expert in Gulf Arab politics and US-Saudi relations at The Washington Institute, says the need for an anti-corruption drive "should not be dismissed."

Nevertheless, she adds: "The scale and scope of the arrests... is unprecedented in recent Saudi history, especially of this type of elite elements. So this is a politically risky move."

MBS' power grab at home is mirrored in his "risky -- some would say brave, some would say rash -- policies in the region," such as the Qatar boycott and doubling down in Yemen.

"He's essentially attempting to wipe out opponents," Boghardt said.

Simon Henderson, another Washington Institute fellow who has worked as a consultant for Arab governments, is also concerned.

"The events are unprecedented and we don't know where they're going to end up and, frankly, I don't think the Saudis know where they're going to end up as well," he said.

Prince Mohammed, Henderson suspects, believes his power play will strengthen his own position and encourage foreign investment. "I think he may be wrong on both counts," he warns.