US Secretary of State Rex Tillerson cautioned countries in the Middle East against using Lebanon as "a venue for proxy conflicts".
Mr Tillerson issued the message, apparently aimed at the Saudis but which could easily include Iran, after leader of the militant group Hezbollah said Riyadh had "declared war" on Lebanon by holding its Prime Minister, Saad Hariri, against his will.
A political crisis has gripped Lebanon and shattered the relative peace maintained by its coalition government ever since his stunning announcement on November 4 from the Saudi capital that he was resigning.
The announcement from the Saudi-aligned Mr Hariri jolted Lebanon and thrust it back into the regional rivalry between Saudi Arabia and Iran.
The move and exceptionally strong statements by the Saudis against Iran that followed have deepened the mystery about Mr Hariri's fate and led to rumours that he is being held in the kingdom against his will, despite his denials.
For the past year, Mr Hariri has headed a coalition government that included members of the Iranian-backed Hezbollah militia.
He cited meddling in Lebanon and elsewhere in the region by Iran and Hezbollah in his decision to step down, adding that Iran's arm into the region will be "cut off."
Saudi Arabia appears to want to see Lebanon headed by someone would form a government without Hezbollah, perhaps believing Mr Hariri has become too lenient toward the group.
If Mr Hariri wants to step down, Mr Tillerson said, he needs to "go back to Lebanon" and formally resign, "so that the government of Lebanon can function properly."
Lebanese President Michel Aoun told Saudi Charge d'Affaires Walid al-Bukhari said on Friday that the manner in which Mr Hariri resigned "was unacceptable," a Lebanese official told The Associated Press, speaking on condition of anonymity.
In a televised speech, Hezbollah leader Hassan Nasrallah said Mr Hariri was being detained in Saudi Arabia and that his "forced" resignation was unconstitutional because it was made "under duress."
"It is clear that Saudi Arabia ... declared war on Lebanon," he said.
Mr Nasrallah said he was certain that Mr Hariri was forced to resign as part of what he called a Saudi policy of meddling in Lebanon's affairs.
Mr Hariri is being prevented by Saudi officials from returning to Lebanon, he said, adding that his detention should not be accepted.
But Tillerson said he's seen "no indication" that Hariri was being held against his will.
An official in French President Emmanuel Macron's office also said Mr Hariri has told foreign ambassadors in Saudi Arabia, where he has been since the resignation announcement, that he is not a prisoner.
The French and US ambassadors met with Mr Hariri, who "says he is not a prisoner, the (Saudi crown) prince says he is not a prisoner," said the official, who spoke on condition of anonymity because he was not authorised to talk to the media.
Mr Macron visited Saudi Arabia on Thursday and met with Crown Prince Mohammed bin Salman to discuss the tensions between the kingdom and Lebanon, a former French protectorate.
The official said Mr Hariri did not ask to see Mr Macron, and French officials "don't have any specific signs" that Mr Hariri's life is in danger.
The crisis was widely seen as a bid by Saudi Arabia to wreck Lebanon's coalition government to try to undermine and limit Iran's influence in the country through the power that Hezbollah wields.
In the first concrete action against Lebanon after days of threats by Saudi government officials, Saudi Arabia and other Gulf countries ordered their citizens to leave the country amid the soaring tensions.