U.S. President Donald Trump on Monday endorsed a move by Saudi Arabia’s future king that tightened his grip on power through the arrests of royals, ministers and investors in an anti-corruption purge.

Axar.az reports citing Reuters that the endorsement cemented a U.S.-Saudi relationship that has improved dramatically under Trump’s presidency, partly because of both leaders’ vision of confronting Riyadh’s arch-rival Iran more aggressively in the region.

Trump tweeted on Monday that he had “great confidence in King Salman and the Crown Prince of Saudi Arabia” following the mass arrests - the biggest such purge of the kingdom’s affluent elite in its modern history.

Trump also tweeted that “they know exactly what they are doing,” adding: “Some of those they are harshly treating have been ‘milking’ their country for years.”

The purge was the latest in a series of dramatic steps by Crown Prince Mohammed bin Salman, or MbS as he is commonly referred to in Western circles, to assert Saudi influence internationally and amass more power for himself at home.

A U.S. official who declined to be named told Reuters that MbS “has become the primary driver of Saudi policy-making. He has moved aggressively to sideline opponents, concentrate decision-making authority, and establish himself as the undisputed heir to the al Saud legacy.”

”He seeks to reinvigorate the public’s confidence in the Saudi monarchy by diversifying the economy, loosening religious restrictions, and carrying out wide ranging social reforms,” the official said.