The United States has declined to comment on the status of Lebanon's former prime minister who suddenly announced his resignation from the capital of Saudi Arabia last week and has yet to return to Beirut.

State Department spokeswoman Heather Nauert told a press briefing on Thursday that Chris Henzel, the US charge d'affaires in Riyadh, met Saad Hariri on Wednesday, but refused to comment on where the meeting took place or to elaborate on Hariri's status.

"[The talks] were sensitive, private, diplomatic conversations," Nauert said.

"We have seen him. In terms of the conditions of him being held or the conversations between Saudi Arabia and Prime Minister Hariri, I would have to refer you to the government of Saudi Arabia and also to Mr Hariri's office."

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When asked by reporters whether the US has a comment regarding the resignation of Hariri, Nauert said it was an "internal matter that we couldn't comment on".

Russia threat

Meanwhile, Russia's ambassador to Lebanon, Alexander Zasypkin, threatened on Thursday to refer Hariri's case to the UN Security Council if the "ambiguity" continues.

"The issue of Hariri's return to the country concerns the sovereign rights of Lebanon," Zasypkin said in an interview with Lebanese channel LBC.

Lebanese officials have said Hariri is likely to be under either house arrest or in temporary detention in Riyadh.

His resignation came on the same day that dozens of Saudi princes, senior ministers, businessmen were arrested in a purge carried out by a new anti-corruption committee led by Crown Prince Mohammed bin Salman.

France's Foreign Minister Jean-Yves Le Drian told Europe-1 radio "to our knowledge" Hariri is not being held by Saudi authorities. Hariri is "free in his movements", he said on Friday, adding "it is up to him to make his choices".

Le Drian's office wouldn't say where France's information came from.

French President Emmanuel Macron discussed Lebanon, a former French colony, during a surprise visit on Thursday to Riyadh.