Lebanon's powerful Hezbollah chief Hassan Nasrallah accused Saudi Arabia of detaining prime minister Saad Hariri and of asking the Shiite movement's arch-foe Israel to launch strikes.
Hariri's sudden resignation on November 4 in a statement made from Riyadh took the Lebanese political class by surprise and has sparked international concern.
The United States warned against using the tiny country as a "venue for proxy conflicts" and the United Nations said it was "essential no new conflict erupt in the region".
France called for Hariri to have "all his freedom of movement" amid rumours he was being held against his will.
"The head of the Lebanese government is detained in Saudi Arabia, he is banned from returning to Lebanon until now," Nasrallah said in a televised address.
Hariri's situation was not completely clear but calls, including from his Lebanese political rivals, mounted for Saudi Arabia to guarantee the premier's freedom of movement.
The 47-year-old announced his resignation on November 4 in a surprise move that coincided with a sweeping purge of the Saudi kingdom's elite, ostensibly over embezzlement accusations.
Hariri, who also holds Saudi nationality, did not say when he would return to Lebanon, where President Michel Aoun has yet to formally accept his resignation.
In a statement issued on Friday after a meeting with a senior Saudi diplomat, Aoun insisted Hariri should return to Lebanon.
"President Aoun met Saudi charge d'affaires Walid Bukhari and informed him that the circumstances in which Mr Hariri's resignation took place were unacceptable," the statement said.
The president "called for the return to Lebanon of the head of the government".
Aoun, whose political ally Hezbollah is a fierce critic of Riyadh, "voiced his concern over what is being said" about Hariri's current status in Saudi Arabia and demanded a "clarification".
US Secretary of State Rex Tillerson on Friday described Hariri as "a strong partner" and warned against "any party, within or outside Lebanon, using Lebanon as a venue for proxy conflicts or in any manner contributing to instability in that country".
"The United States strongly supports the sovereignty and independence of the Republic of Lebanon and of its political institutions" and opposes "any actions that could threaten that stability," he said.
French President Emmanuel Macron, whose country has close ties with both Lebanon and Saudi Arabia, made a surprise visit to Riyadh late Thursday after a trip to the United Arab Emirates.
On Friday, a spokesman for France's foreign ministry said: "We wish Mr. Saad Hariri to have all his freedom of movement and to be fully able to play the essential role that is his in Lebanon."
Earlier French Foreign Minister Jean-Yves Le Drian said on French radio he thought Hariri was "free to move around", despite most of the Lebanese political class implying he was de facto under house arrest.
"He went to Abu Dhabi the day before President Macron's visit (on Wednesday) so we think he's free to move around," Le Drian said.
Nasrallah, whose party is the only organisation that did not disarm after the 1975-1990 civil war and now has an arsenal that outstrips Lebanon's own armed forces, accused Saudi Arabia of seeking to incite conflict.
"The most dangerous thing is inciting Israel to strike Lebanon," he said. "I'm talking about information that Saudi Arabia has asked Israel to strike Lebanon."
Nasrallah, whose movement Hariri has repeatedly said should lay down its arms, also warned that his movement, which fought a devastating war with Israel in 2006, was stronger than ever.
"We are stronger today, we warn them again misguided calculations, against any knee-jerk initiative," he said, adding however that his party saw any Israeli attack as being unlikely at this stage.
The UN chief, Antonio Guterres, also said: "It is essential that no new conflict erupt in the region."
"We are indeed very worried and we hope that we won't see an escalation in the region that would have tragic consequences," he said.
Lebanon's interior ministry said in a statement Friday that a Saudi national had been kidnapped in Lebanon in circumstances that remained unclear.
Saudi Arabia had urged all its citizens in Lebanon this week to leave the country but Interior Minister Nohad Machnouk told AFP that the kidnapping was "in no way related" to the Hariri crisis or politics.