Saudi Arabia's mass purge of elite figures "raises a few concerns", US Secretary of State Rex Tillerson said Friday, reacting with caution to the crackdown on graft and also over tensions in Lebanon.

The unprecedented roundup has seen more than 200 princes, ministers and businessmen detained over what Riyadh alleges is $100 billion in embezzlement, but what is also widely viewed as a move by Crown Prince Mohammed bin Salman to consolidate power ahead of his accession to the throne.

The upheaval comes as Riyadh is locked in an intensifying proxy war with regional rival Tehran and enforces a crippling aid blockade of Yemen, which the United Nations has warned could trigger the world's worst famine in decades.

Tillerson, 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 Trump, who said he had "great confidence" in what the crown prince and his father King Salman were doing.

Saudi authorities have frozen the bank accounts of those accused and warned that assets related to the alleged corruption cases would be seized as state property.

The crackdown comes as the young crown prince moves to accelerate his Vision 2030 programme to modernise the conservative kingdom, but also as Riyadh takes a more aggressive stance toward Iran.

- Saudi blockade, Yemen 'setback' -

The World Health Organization (WHO) warned Friday that a Saudi-led blockade of Yemen was threatening to undo efforts to rein in a cholera epidemic already affecting nearly one million people in the war-ravaged country.

Almost 2,200 people have so far died from the waterborne disease, which has propagated rapidly due to deteriorating hygiene and sanitation conditions.

WHO spokeswoman Fadela Chaib warned that the Saudi-led coalition's decision to seal off Yemen's borders threatened progress made in fighting the epidemic.

"We will suffer a major setback if we don't have full access to all affected areas," she told reporters in Geneva.

The coalition imposed a blockade on all aid deliveries to rebel-held territory in the wake of a failed missile attack against Riyadh airport one week ago by Yemen's Iran-backed Huthi rebels.

Iran vehemently denied a Saudi charge that it supplied missiles to the Huthis but said they were justified in responding after years of bombardment.

The UN Security Council demanded on Wednesday that the blockade be lifted but there was still no response from the coalition as aid deliveries were blocked for a fifth day on Friday.

- Warning of new coalition -

Lebanon has also become increasingly embroiled in the regional rivalry between Riyadh and Tehran, prompting Tillerson to step in on Friday and urge all parties not to use the country to settle their differences.

"The United States cautions against any party, within or outside Lebanon, using Lebanon as a venue for proxy conflicts or in any manner contributing to instability in that country," Tillerson said.

International concern over Lebanon also sparked a warning from the UN chief that a new regional conflict must be averted.

Secretary General Antonio Guterres said he was "very worried" a conflict could break out in Lebanon and that he was engaged in intense contacts with all players to urge de-escalation.

"It is essential that no new conflict erupt in the region," Guterres told reporters. "It would have devastating consequences."

Lebanon's prime minister Saad Hariri resigned in a shock move from the Saudi capital last week, citing Iran's "grip" on his country and threats to his life.

But the head of Lebanon's powerful Hezbollah movement on Friday accused Saudi Arabia of holding Hariri hostage against his will.

"The head of the Lebanese government is detained in Saudi Arabia, he is banned from returning to Lebanon until now," Nasrallah said in a televised address.

The Sunni leader had been sharing power with the Iran-backed Shiite militant movement Hezbollah in a hard-won national unity government.

But the arrangement had come under increasing fire from Saudi Arabia, where he holds dual citizenship.

Source: AFP