Egyptian president Mohamed Morsi has cut all diplomatic ties with Syria and expressed support for a no-fly zone over the war-torn country.

Mr Morsi made the announcement at a rally in Cairo held in support of the Syrian opposition.

"We decided today to entirely break off relations with Syria and with the current Syrian regime," he said.

He also urged world powers to enforce a no-fly zone and warned allies of the Syrian president, Bashar al-Assad, to pull back.

"Hezbollah must leave Syria... there is no space or place for Hezbollah in Syria," said Mr Morsi.

He said the Syrian embassy in Cairo will be closed and Egypt's representative withdrawn from Damascus.

Mr Morsi waved Syrian and Egyptian flags in front of the auditorium packed with 20,000 supporters.

"From the free revolutionaries of Egypt, we will stamp on you, Bashar," the crowd chanted.

Mr Morsi also flagged a summit of Arab and other Islamic states to discuss the crisis.

He said he had made "contact with Arab and Muslim states to organise an emergency support meeting" for the Syrian people.

In recent days the US has announced it will take steps to arm the rebels after finding enough evidence to prove the Syrian regime had employed the use of chemical weapons.

Western diplomats said that Washington was considering a limited no-fly zone over parts of Syria.

But the White House later said it had no national interest in pursuing that option.

The Egyptian army is funded and trained by the US and is one of the most powerful in the Middle East, but there has been no suggestion that Egypt should join the fighting.

Russia hits back at threat of no-fly zone

Russia, an ally of Mr Assad and fierce opponent of outside military intervention in Syria, said any attempt by the US to enforce a no-fly zone over Syria would violate international law.

A no-fly zone is among of range of military support options reportedly being considered by the U-S in order to assist the rebels.

The US has moved fighter jets and missiles to Jordan, but it says this is merely part of an annual exercise.

Russia says America's evidence of chemical weapons use does not meet stringent criteria for reliability.

Russian foreign minister Sergei Lavrov says he does not believe American intelligence.

"What would be the sense of the regime using chemical weapons, moreover on such a small scale, to expose itself?" he said.

"From a military point of view it is nonsensical and everybody understands that.

"Until recently [the US] said that they are not going to provide military support to the opposition if the regime doesn't cross the red line.

"But the report of the supposed use of chemical weapons, in our view, in our assessment, completely does not reach the red line."

The uprising broke out in Syria in March 2011 and has turned into a bloody civil war.

The United Nations says more than 93,000 people have died in the conflict.

ABC/wires