Egyptian President Mohamed Mursi said he had cut all diplomatic ties with Damascus on Saturday 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, 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.
"There is no space or place for Hezbollah in Syria," Mursi said.
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 presidential 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.
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.
Egypt's U.S.-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.
WARNS AGAINST VIOLENCE
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 this week from Sunni Muslim religious organisations for jihad against Assad and his Shi'ite allies.
It marked Mursi's second combative foreign policy speech in less than a week. On Monday, he said Egypt would keep "all options open" for dealing with a dispute with Ethiopia over a giant dam it is building on the Nile, though he said Cairo did not want war and stressed it would work diplomatically.
Mursi's liberal and leftist opponents are mobilising for mass protests on June 30, the anniversary of Mursi coming to office, fuelling fears of possible further violence.
Mursi told his Islamist supporters at the rally that they must not be dragged into confrontations and that he would not tolerate any violence. Additional reporting by Ali Abdellati