Lebanon's Prime Minister Saad Hariri announced his resignation Saturday, claiming that it stemmed from a "rejection of Iran's political over-extension in the region".
"Wherever Iran settles, it sows discord, devastation and destruction, proven by its interference in the internal affairs of Arab countries", Hariri said, adding that Irans "hands" in the region "will be cut off".
"The evil that Iran spreads in the region will backfire on it", he said Saturday.
"We are living in a climate similar to the atmosphere that prevailed before the assassination of (his father the late prime minister) martyr Rafik al-Hariri".
"I sensed whats being woven in secret to target my life", he said.
An official at the US State Department said it was following the situation closely and noted that Hariri had been a "strong partner in building strong national state institutions and in the war on terror".
Hezbollah has sent thousands of its fighters to Syria to shore up President Bashar al-Assad's government.
Mr Hariri's father, former prime minister Rafik al-Hariri, was assassinated in 2005.
That raised fears of a new conflict in the country, still scarred by its 1975-1990 civil war.
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Walid Jumblatt, the leader of Lebanon's Druze minority, said Lebanon was too weak to bear the consequences of Hariri's resignation, saying he feared political and economic fallout.
They said they were now waiting for Mr Hariri to return to Lebanon to clarify the circumstances of his resignation and proceed accordingly.
Al-Sabhan met with Hariri in Saudi Arabia when the now resigned prime minister was visiting earlier this week.
"The Saudi Arabia-Iranian rivalry now is going to play out on Lebanon's streets in the next few days and next few weeks".
At his family's urging, he returned to Lebanon to enter politics, playing a key role in mass demonstrations that ended with the departure of Syrian forces from Lebanon after a 30-year presence.
Lebanese political analyst Hilal Khashan argued that Saudi Arabia had been piling the pressure on its protege lately and "summoned" him to Riyadh.
"There is a real danger that the tensions between the Hariri-dominated faction and Hezbollah-dominated faction could easily escalate into an armed confrontation", said Gerges, who is chair of contemporary Middle Eastern studies at London School of Economics.
It was not immediately clear whether Hariri meant to return to Lebanon.
Aoun's office said Hariri had called him from "outside Lebanon" to inform him of his resignation.