Lebanon's president has accused Saudi Arabia of having "detained" Prime Minister Saad Hariri and using a notorious private security contractor to guard him.

Michel Aoun said on Wednesday that Blackwater personnel were securing Hariri, who announced his resignation in a statement from the kingdom last week, prompting speculation that he has been detained along with dozens of Saudis in an anti-corruption purge.

"Lebanese authorities have unconfirmed information that the Blackwater firm is guarding Hariri and his family - not official Saudi security forces," Aoun said in a tweet before it was later deleted.

The Lebanese President's remark comes after anonymous Saudi commentator who uses the Twitter handle al-Ahd al-Jadid (The New Era) alleged that Saudi Crown Prince Mohammed bin Salman has hired the mercenaries to watch over the previous crown prince Mohammed bin Nayef, who was ousted in June.

The microblogger, who is closely associated to notorious whistle-blower Mujtahidd, said despite bin Salman's powerful position he has lost confidence in local security forces and even employed the guns-for-hire as his own personal bodyguards.

"The first group of Blackwater mercenaries arrived in Saudi Arabia a week after the toppling of bin Nayef. They were around 150 fighters. Bin Salman sent some of them to secure bin Nayef's place of detention and the rest he used for his own protection," the commentator said.

He added that Crown Prince of Abu Dhabi Mohammed bin Zayed set up the deal with the security firm worth an alleged $1 billion for over a thousand hired troops.

Blackwater military contractors killed 17 unarmed Iraqi civilians and injured 20 in a notorious 2007 massacre in Baghdad that prompted the firm to change its name.

The Saudi-led coalition fighting rebels in Yemen has increasingly used private military corporations such as Blackwater.

Blackwater, which is based in North Carolina, is now known as Academi, having previously been sold and renamed as Xe Services.

Reports first emerged in 2015 that the UAE was sending mercenaries to fight in Yemen, choosing not to send its own citizens to take part the war against the Houthi rebels.

The UAE has been accused of running secret prisons in Yemen where torture was said to be widely used against detainees.

Last month, Qatar's former deputy prime minister accused the UAE of plotting to invade Doha with an army of mercenaries.