Hezbollah leader Sayyed Hassan Nasrallah says Saudi Arabia has declared war on Lebanon and Hezbollah, adding that Lebanese Prime Minister Saad al-Hariri, who resigned in a speech from Riyadh on Saturday, was being detained by the Saudis.

In a televised address, Nasrallah said Hariri's resignation was an "unprecedented Saudi intervention" in Lebanese politics and called for Hariri to return to Lebanon. He said Lebanon's government was still legitimate and had not resigned.

"Let us say things as they are: the man is detained in Saudi Arabia and forbidden until this moment from returning to Lebanon," Nasrallah said.

"It is clear that Saudi Arabia and Saudi officials have declared war on Lebanon and on Hezbollah in Lebanon," he added.

Since Hariri's announcement, Saudi Arabia has accused Lebanon and its Shia Hezbollah movement of declaring war on it. Riyadh has advised Saudi citizens not to travel to Lebanon, or if already there to leave as soon as possible. Other Gulf states have also issued travel warnings.

Meanwhile, Lebanese President Michel Aoun told Saudi Arabia's envoy on Friday that Hariri must return, presidential sources said.

'Go back to Lebanon'

U.S. Secretary of State Rex Tillerson said if Hariri wants to step down, he needs to "go back to Lebanon" and formally resign, "so that the government of Lebanon can function properly."

But Tillerson, speaking to reporters Friday while flying from China to Vietnam, 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 Hariri has told foreign ambassadors in Saudi Arabia, where he has been since the resignation announcement, that he is not a prisoner.

Saudi Crown Prince Mohammed bin Salman, right, meets with French President Emmanuel Macron upon his arrival in Riyadh, Saudi Arabia on Thursday. (Saudi Press Agency/Associated Press)

The French and U.S. ambassadors met with Hariri, who said 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 authorized to talk to the media.

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 Hariri did not ask to see Macron, and French officials "don't have any specific signs" that Hariri's life is in danger.

French Foreign Minister Jean-Yves Le Drian told Europe-1 radio that "to our knowledge," Hariri is not being held against his will, adding that France believes "he is free in his movements, and it is up to him to make his choices."

Held in Saudi Arabia?

The Lebanese authorities believe Hariri is being held in Saudi Arabia, two top Lebanese government officials, a senior politician close to Hariri and a fourth source told Reuters on Thursday, amid a deepening crisis pushing Lebanon onto the frontlines of a power struggle between Saudi Arabia and Iran.

On Thursday, Hariri's Future Movement political party said his return home was necessary to uphold the Lebanese system, describing him as prime minister and a national leader. Aoun has refused to accept the resignation until Hariri returns to Lebanon to deliver it to him in person and explain his reasons.

The Lebanese government said Friday that president Michel Aoun, left, pressed Saudi Arabia's envoy for the return of Prime Minister Saad al-Hariri, right. (Dalati Nohra/EPA)

Riyadh says Hariri, a long-time Saudi ally, is a free man and that it had nothing to do with his decision to announce his resignation on Saturday while in Saudi Arabia.

The conflict has raised concern that Riyadh could take measures against Lebanon, which hosts 1.5 million Syrian refugees. Lebanon, where Sunnis, Shia, Christians and Druze, all backed by rival regional powers, fought a civil war from 1975-1990, maintains a governing system designed to ensure each group is represented.

Saudi-Iran rivalry

Saudi Arabia considers Iranian-allied Hezbollah to be its enemy in conflicts across the Middle East, including Syria and Yemen. The Saudi foreign minister accused Hezbollah of a role in the launching of a ballistic missile at Riyadh from Yemen on Saturday.

Saudi Arabia's Crown Prince Mohammed bin Salman said Iran's supply of rockets to militias in Yemen was an act of "direct military aggression" that could be an act of war. The resignation of Hariri, who as well as a politician is a business tycoon with major investments in Saudi Arabia, also comes as Riyadh has rounded up dozens of senior princes and businessmen in a corruption investigation.

UN Secretary-General Antonio Guterres on Friday said he was "very worried" by the escalating tensions between the Saudis and Lebanon, and that it was essential to avert new conflict.

The young Mohammed bin Salman, known as MBS, has been exerting his influence in Saudi Arabia and the region. (Presidency Press Service/Pool Photo via AP)

"It is essential that no new conflict erupts in the region. It could have devastating consequences," the United Nations chief told reporters.

Tillerson had a more stern message, warning other countries and groups against using Lebanon as vehicle for a larger proxy fight in the Middle East, saying the United States strongly backed Lebanon's independence.

"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 U.S. State Department.

An "international support group" of countries concerned about Lebanon, which includes the U.S., 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. During the meeting with the Saudi envoy, Aoun expressed concern over reports about Hariri's circumstances and urged clarification, presidential sources said.

Hariri, whose father, a long-serving prime minister, was killed by a bomb in 2005, said in his resignation that he feared assassination and blamed Iran for meddling in Lebanon's affairs. His resignation unravelled a political deal among rival factions that made him prime minister and Aoun, a political ally of Hezbollah, head of state last year.

Top Lebanese Druze politician Walid Jumblatt said on Friday it was time that Hariri returned to Lebanon. After a week of absence, "be it forced or voluntary," it was "time for Sheikh Saad to return," Jumblatt said on Twitter.

"By the way, there is no alternative to him," he added.