'Saudi Arabia, US ordered Hariri resignation,' says advisor to Iranian FM

US President Donald Trump and Saudi Crown Prince Mohammad Bin Salman ordered Lebanese Prime Minister Saad Hariri to announce his resignation from Saudi Arabia, a senior advisor to the Iranian foreign minister claimed on Saturday.

"Hariri's resignation was done in coordination with Trump and Mohammed bin Salman to foment tension in Lebanon and the region," Hossein Sheikholeslam said, , according to the semi-official Fars news agency.

It added that Hariri should have resigned from Lebanon and not while in Saudi Arabia.

Earlier on Saturday, Hariri announced his resignation during a trip to Saudi Arabia in a surprise move that has plunged the country into uncertainty amid heightened regional tensions. 

"I announce my resignation from the post of prime minister," Hariri said, citing Iran's "grip" on the country and threats to his life.

"I felt what was being covertly plotted to target my life," Hariri said.

The two-time prime minister, whose father Rafic was assassinated when he held the same position in 2005, accused Iran and its powerful Lebanese Shia ally Hizballah of seeking hegemony in the region.

The 47-year-old Sunni politician's resignation comes less than a year after his government, to which Hizballah's political wing belongs, was formed.

"Iran has a grip on the fate of the region's countries... Hizballah is Iran's arm not just in Lebanon but in other Arab countries too," he said.

"In recent years, Hizballah has used the power of its weapons to impose a fait accompli," he said, reading a speech from behind a desk.

Hizballah is a vital ally of President Bashar al-Assad in Syria against the armed opposition and the Islamic State group.

It enjoys broad support from Iran and is the only Lebanese party to have kept its weapons after the 1975-1990 civil war.

Its arsenal has since grown exponentially and now outstrips that of the nation's own armed forces. 

It claims it is the only credible rampart against neighbouring Israel and its refusal to disarm is the main political crux in Lebanon.

Hizballah members have been accused over the 2005 assassination in a massive car bomb blast of Rafik Hariri, the dominant figure of Lebanon's post-war political landscape.

He made his fortune in Saudi Arabia, where his son Saad was born. Riyadh is Iran's main regional rival and the two powers' tussle for influence has played out in ongoing conflicts in Iraq, Syria and Yemen.

Agencies contributed to this report.