The US, Saudi Arabia and Israel share concern about expanding Iranian influence in the region.
Hezbollah, Iran's Lebanon-based proxy, now controls significant territory in Syria, near its border with Israel, and in Lebanon. While some Israeli officials have voiced support for moving to constrain Hezbollah, others have urged caution.
Saudi allies such as Egypt have strongly opposed military action against Iran or Hezbollah.
Steps leading to the current crisis began last Saturday, when the Saudis accused Iran and Hezbollah of carrying out an "act of war" with a missile they said was fired at Riyadh by Iranian-backed Houthi rebels in neighbouring Yemen.
On the same day, Saudi-backed Lebanese Prime Minister Saad Hariri, who shares power with Hezbollah in an uneasy coalition government, suddenly appeared in the Saudi capital and abruptly announced his resignation from office. The announcement threw Lebanon into confusion and raised fears of war.
On Thursday, Saudi Arabia, Bahrain and Kuwait ordered their citizens out of Lebanon, saying their safety was at risk. The Saudis have suggested Hariri was escaping a Hezbollah assassination plot.
In Lebanon, political leaders from differing factions called on Hariri to return and address his political situation from Beirut.
Hezbollah, which is both a militant group and a political force, has called Hariri's resignation illegal because it was done from afar. Hezbollah leader Hassan Nasrallah said the Saudis had kidnapped Hariri and "asked Israel to attack Lebanon".
Hezbollah has expanded its political role in the complex Lebanese coalition during Hariri's 11 months in office. Hariri's abrupt arrival in Saudi Arabia is seen by many in Lebanon as a blunt signal from Riyadh that he had not done enough to rebuff Hezbollah and Iranian influence, and that Saudi Arabia intends to assert its influence in Lebanon against Iran.
US and European diplomats have met with Hariri in Riyadh, but a senior administration official, asked if Hariri was free to leave Saudi Arabia, said, "We don't know."
Alongside the Hariri drama, Saudi authorities also announced the arrest last Saturday of more than 200 princes, senior officials and prominent Saudi business executives.
Cast as part of a domestic anti-corruption drive, the arrests also left Saudi Crown Prince Mohammed bin Salman in undisputed control of Saudi security services. That may be an attempt to consolidate power before eventually inheriting the throne.
