Hezbollah's leader Hassan Nasrallah said on Friday that Saudi Arabia had declared war on Lebanon and the Iran-backed group, accusing Riyadh of detaining Lebanese Prime Minister Saad Hariri and forcing him to resign.

Nasrallah said Saudi Arabia's detention of Hariri was an insult to all Lebanese and he must return to Lebanon.

Nasrallah also said Riyadh is inciting Israel to strike Lebanon, and his militant group is watching carefully for any Israeli attempts to use the crisis to begin hostilities against Lebanon. He said Israel is cautious, however, and unlikely to make such a move.

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In a televised speech, Nasrallah said "Let us say things as they are: the man is detained in Saudi Arabia and and forbidden until this moment from returning to Lebanon." Nasrallah called Hariri's resignation last Saturday an "unprecedented Saudi intervention" in Lebanese politics.

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

He said Lebanon's government was still legitimate and had not resigned, saying Hariri's "forced" resignation is unconstitutional because it was done "under duress."

"Saudi will fail in Lebanon as it has failed on all fronts," Nasrallah said.

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Nasrallah is calming an apparently jittery population following Saudi Arabia's escalation against Hezbollah's patron Iran. Hariri's resignation last week was seen as a move by Riyadh to take its rivalry with Iran to the tiny Lebanon.

Some fear the escalation will pave the way for Israel to strike against Hezbollah, against which Israel has fought a number of wars. He said Israel should not think "we are troubled. No, absolutely not."

Nasrallah said Saudi Arabia was encouraging Israel to attack Lebanon. While an Israeli attack could not be ruled out entirely, he said, it was unlikely partly because Israel knew it would pay a very high price. "I warn them against any miscalculation or any step to exploit the situation," he said.

"Today we are more confident and feeling stronger in the face of any threat," said Nasrallah.

Western countries have looked on with alarm at the rising regional tension.

U.S. Secretary of State Rex Tillerson warned other countries and groups against using Lebanon as vehicle for a larger proxy fight in the Middle East, saying Washington strongly backed Lebanon's independence and respected Hariri as a strong partner of the United States, referring to him as prime minister.

"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.

The French foreign ministry said it wanted Hariri to be fully able to play what it called his essential role in Lebanon.

Hariri has made no public remarks since announcing his resignation in a speech televised from Saudi Arabia, saying he feared assassination and accusing Iran and Hezbollah of sowing strife in the Arab world.

Two top Lebanese government officials, a senior politician close to Hariri and a fourth source told Reuters on Thursday that the Lebanese authorities believe Hariri is being held in Saudi Arabia.

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. Those steps have raised concern that Riyadh could take measures against the tiny Arab state, which hosts 1.5 million Syrian refugees.