United Nations Secretary-General Antonio Guterres warned today that a new conflict in Lebanon would have "devastating consequences" and said he was engaged in intense contacts with all players to urge de-escalation.
Lebanon's Prime Minister Saad Hariri resigned in a shock announcement broadcast from Riyadh as a power struggle intensified between Saudi Arabia and Iran, which backs Lebanon's Hezbollah movement.
Hezbollah's leader Sayyed Hassan Nasrallah said on Friday that Saudi Arabia had declared war on Lebanon and Hezbollah, accusing Riyadh of detaining Saad al-Hariri and forcing him to resign to destabilise the country.
France became the first Western country to indicate that Saudi Arabia was holding Hariri against his will, saying it wished for him to have "all his freedom of movement and be fully able to play the essential role that is his in Lebanon".
Hariri's resignation has plunged Lebanon into crisis, thrusting the small Arab country back to the forefront of regional rivalry between the Sunni Muslim monarchy of Saudi Arabia and Shia revolutionary Iran.
US Secretary of State Rex Tillerson warned other countries and groups against using Lebanon as vehicle for a larger proxy fight in the Middle East, saying Washington strongly backed Lebanon's independence and respected Hariri as a strong partner of the United States, still referring to him as prime minister.
"There is no legitimate place or role in Lebanon for any foreign forces, militias or armed elements other than the legitimate security forces of the Lebanese state," Tillerson said in a statement released by the US State Department.
Tillerson told reporters on Friday there was no indication that Hariri was being held in Saudi Arabia against his will but that the United States was monitoring the situation.
The French foreign minister said earlier on Friday that he also believed Hariri was a free man - a statement at odds with the later French foreign ministry comment that it wanted Hariri to have "all his freedom of movement".
Hariri has made no public remarks since announcing his resignation in a speech televised from Saudi Arabia, saying he feared assassination and accusing Iran and Hezbollah of sowing strife in the Arab world.
Two top Lebanese government officials, a senior politician close to Hariri and a fourth source told Reuters on Thursday that the Lebanese authorities believe Hariri is being held in Saudi Arabia.
Hezbollah chief Nasrallah said Saudi Arabia was encouraging Israel to attack Lebanon. While an Israeli attack could not be ruled out entirely, he said, it was unlikely partly because Israel knew it would pay a very high price. "I warn them against any miscalculation or any step to exploit the situation," he said.
"Saudi will fail in Lebanon as it has failed on all fronts," Nasrallah said.
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.
Hariri's resignation unravelled a political deal among rival factions that made him prime minister and President Michel Aoun, a political ally of Hezbollah, head of state last year.
The coalition government included Hezbollah, a heavily armed military and political organization.
Aoun told Saudi Arabia's envoy on Friday that Hariri must return to Lebanon and the circumstances surrounding his resignation as prime minister while in Saudi Arabia were unacceptable, presidential sources said.
An "international support group" of countries concerned about Lebanon, which includes the United States, Russia and France, appealed for Lebanon "to continue to be shielded from tensions in the region". In a statement, they also welcomed Aoun's call for Hariri to return.
Riyadh says Hariri is a free man and he decided to resign because Hezbollah was calling the shots in his government. Saudi Arabia considers Hezbollah to be its enemy in conflicts across the Middle East, including Syria and Yemen.