Egyptian President Mohammed Mursi said he had cut all diplomatic ties with Damascus and backed a no-fly zone over Syria, pitching the most populous Arab state more firmly against President Bashar al-Assad.
Addressing a rally called by Sunni Muslim clerics in Cairo on Saturday, the Sunni Islamist head of state also warned Assad's ally, the Iranian-backed Lebanese Shi'ite militia Hezbollah, to pull back from fighting in Syria.
"Hezbollah must leave Syria. These are serious words," said Mursi, whose country hosted a conference of Sunni clerics who called for a holy war against Damascus. "There is no space or place for Hezbollah in Syria."
The rally underscored the region's deepening sectarian rift.
A cleric who spoke before Mursi described Shi'ites as heretics, infidels, oppressors and polytheists. It was also a show of support for Mursi as his opponents mobilise for protests to demand early elections.
Mursi waved Syrian and Egyptian flags as he entered the auditorium packed with 20,000 supporters. The crowd chanted: "From the free revolutionaries of Egypt: We will stamp on you, Bashar!"
Mursi, a Muslim Brotherhood politician, steered clear of direct references to Shi'ites and Iran but in a partial allusion to Tehran, he accused states in the region and beyond of feeding "a campaign of extermination and planned ethnic cleansing" in Syria.
"We decided today to entirely break off relations with Syria and with the current Syrian regime," he said. He also urged world powers not to hesitate to enforce a no-fly zone over Syria.
Western diplomats said on Friday that Washington was considering a limited no-fly zone over parts of Syria, but the White House said later that the United States had no national interest in pursuing that option.
Russia, an ally of Assad and opponent of outside military intervention in Syria, said any attempt to impose a no-fly zone using F-16 fighter jets and Patriots based in Jordan would be illegal.
Mursi said he was organising a summit of Arab and other Islamic states to discuss the situation in Syria, where the United States has in recent days decided to take steps to arm the rebels.
Egypt's US-funded and -trained army is among the most powerful in the Middle East. There has been no suggestion, however, that Egypt, a country steeped in poverty, should get involved in the fighting in Syria.
Mursi said: "The Egyptian people supports the struggle of the Syrian people, materially and morally, and Egypt, its nation, leadership ... and army, will not abandon the Syrian people until it achieves its rights and dignity."
The Brotherhood has joined calls from Sunni Muslim organisations for jihad against Assad and his Shi'ite allies.
Egypt has not taken an active role in arming Syrian rebels, but a Mursi aide said that Cairo would not stand in the way of Egyptians who wanted to fight in Syria.