- Video report by ITV News International Affairs Editor Rageh Omaar
US Secretary of State Rex Tillerson has warned against the Middle East using Lebanon as "a venue for proxy conflicts".
Mr Tillerson's message comes after Lebanon's Hezbollah leader claimed Saudi Arabia was forcing the Lebanon's Prime Minister to resign and detaining him.
Hassan Nasrallah said Saudi Arabia had carried out these acts to destabilise Lebanon.
Saad Hariri resigned on Saturday in a televised address from Riyadh saying the reason was because he feared he was at risk of being assassinated.
In his resignation speech, Mr Hariri also accused Iran and its Hezbollah allies of spreading conflict and division.
Mr Nasrallah continued that Mr Hariri is being prevented by Saudi officials from returning to Lebanon, and said his detention should not be accepted.
France was the first Western country to indicate that Hariri was being held in Saudi Arabia against his will, saying it hopes for him to have “all his freedom of movement and be fully able to play the essential role that is his in Lebanon”.
Though this was strangely contradictory to an earlier statement by the French foreign minister who said that he believed Hariri was a free man.
The Hezbollah Secretary General's calls were echoed by Lebanese officials who also called for the return home of their Prime Minister.
In a televised speech the Hezbollah leader said: “Let us say things as they are: the man is detained in Saudi Arabia and forbidden until this moment from returning to Lebanon”.
“It is clear that Saudi Arabia and Saudi officials have declared war on Lebanon and on Hezbollah in Lebanon,” he said.
Riyadh reflected the accusation on Monday saying that Lebanon and Hezbollah had declared war on the conservative Gulf Arab kingdom.
Riyadh says Hariri is a free man and he decided to resign because Hezbollah was dictating the government.
US Secretary of State Rex Tillerson warned other countries and groups against using Lebanon for a larger proxy fight in the Middle East
He said Washington strongly backed Lebanon’s independence and respected Hariri as a strong partner of the United States.
“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,” in a statement released by the US State Department, Tillerson said.
Tillerson told reporters there was no indication that Hariri was being held in Saudi Arabia against his will but that the United States was monitoring the situation.
Since Mr Hariri's shock announcement on November 4, a political crisis has gripped Lebanon and shattered the relative peace maintained by its coalition government.
Saudi Arabia supporting Mr Hariri's announcement has thrust Lebanon back into the regional rivalry between Saudi Arabia and Iran, leaving it caught between the Shia Muslim nation of Iran and the Sunni puritanical kingdom of Saudi Arabia.
For more than two years, Iran and Saudi Arabia have been locked in a proxy civil war in Yemen, the poorest country in the Arab world.
Last week the war threatened to spill over when Iranian-backed Houthi rebels in Yemen fired a missile at the Saudi capital Riyadh, which the United States claimed bore "all the hallmarks" of having been made in Iran.
Meanwhile in Syria, Hezbollah fighters backed by Iran have been making advances on behalf of President Bashar Al-Assad, in turn pushing back rebel groups backed by Saudi Arabia.
In recent days the group's soldiers were pictured holding up the flags of Hezbollah and Syria side by side.
Following Mr Hariri's resignation in Saudi Arabia, and its destabilising effect on the political situation in his country, there could be a new front in the regional power struggle: Lebanon.
To deepen the political crisis, Saudi Arabia is currently in the grip of an apparent power grab by Crown Prince Mohammad Bin Salman.
"Mohammad Bin Salman has said in the past that Saudi Arabia should take the confrontation to Iran and he says that is what they're doing in Yemen, although militarily they have not been so successful," explained Jane Kinninmont, a Middle East expert for Chatham House explained.
"Saad Hariri the Lebanese PM resigned from Riyadh and has not been back to the country since then, and that has made many people think that he was possibly coerced or presuaded by Saudi Arabia - rather than simply making this decision himself."
Lebanon is a popular holiday destination for many Saudi citizens as it allows them to escape the strict Islamic codes back home, but holidaymakers have been urged to leave the country by their government, another example of how Riyadh is exerting pressure on Lebanon.
The latest developments in the area made Lebanon a new potential flash point in the region, with Hezbollah reportedly pulling fighters out of Syria should a new proxy war break out between Iran and Saudi Arabia.