Regional tensions over Lebanon are escalating, after Hezbollah's leader says Saudi Arabia has declared war on Lebanon.

It comes as the leader of the Iran backed group accused Riyadh of detaining Lebanon's prime minister Saad al-Hariri and forcing him to resign.

A senior politician close to Hariri has told Reuters he was forced to do it and that the Lebanese authorities believe Hariri is being held in Saudi Arabia.

But Riyadh says Hariri is a free man and he decided to resign because Hezbollah was calling the shots in his government.

It also accused Lebanon and Hezbollah of declaring war on the Gulf Arab Kingdom on Monday.

Hariri's resignation has plunged Lebanon into turmoil, pushing the small Arab country back to the forefront of regional rivalry between the Sunni Muslim monarchy Saudi Arabia and Shi'ite revolutionary Iran.

Western countries are looking on with alarm at the rising regional tension.

US Secretary of State Rex Tillerson warned the region against using Lebanon as a vehicle for a larger proxy in the Middle East on Friday.

And said there was no indication that Hariri was being held in Saudi Arabia against his will.

While France's President Macron has called on Lebanon's President to discuss the developments of the shock resignation.

Lebanon's Prime Minister has made no public remarks since the announcement in a speech televised from Saudi Arabia last week, saying he feared assassination.

Hariri's resignation is being widely seen as part of a Saudi attempt to counter Iran as its influence deepens in Syria and Iraq and as Riyadh and its allies battle Iranian-allied Houthi rebels in Yemen.