The US Secretary of State has warned other countries against using the crisis in Lebanon as a "venue for proxy conflicts", as the resignation of the country's Prime Minister is stoking tensions in the region.
Saad Hariri, the Lebanese Prime Minister, announced from Saudi Arabia that he was resigning, plunging the country into political crisis.
The leader of the Lebanese militia Hezbollah has accused Saudi Arabia of detaining Mr Hariri against his will and forcing him to step down to destabilise the country.
Hezbollah leader Sayyed Hassan Nasrallah said Saudi Arabia had declared war on Lebanon.
US Secretary of State Rex Tillerson said Mr Hariri should return to the country to make his resignation "official".
"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," he said in a statement.
Mr Tillerson added that there was no indication that Mr Hariri was being held against his will but that the US was monitoring the situation.
Mr Hariri has made no public remarks since announcing his resignation in a speech televised from Saudi Arabia, which says it is protecting him from an assassination plot.
Mr Nasrallah, the Hezbollah leader, claimed Mr Hariri's resignation was an "unprecedented Saudi intervention" and called for his return as he insisted Lebanon's government was still legitimate.
He said the "insult" to Mr Hariri was an insult to all Lebanese people, and said Saudi Arabia was inciting Israel to strike Lebanon.
But Mr Nasrallah said he thought a war with Israel would be unlikely, before warning the country against any attempt to exploit the ongoing tensions with Saudi Arabia.
Tensions in the region continued to intensify this week as Saudi Arabia ordered its citizens visiting or living in Lebanon to leave the country.
The UN's Secretary General Antonio Guterres said he was "very worried" by the growing friction between Saudi Arabia and Lebanon.
He said: "It is essential that no new conflict erupts in the region. It could have devastating consequences."Let's