Egypt's Islamist President Mohammed Morsi vowed today not to allow further attacks on foreign embassies in Cairo, saying the Egyptian people reject such "unlawful acts".

Speaking during a visit to the European Union in Brussels, Mr Morsi said he had spoken to President Barack Obama and condemned "in the clearest terms" the Tuesday attacks on the US consulate in the Libyan city of Benghazi in which the ambassador and three other Americans died.

Crowds protesting at the US Embassy in Cairo the same day climbed its walls and tore down an American flag, which they replaced briefly with a black, Islamist flag.

Officials were investigating whether the Libya rampage was a backlash to an anti-Islamic video with ties to Coptic Christians or a plot to coincide with the anniversary of the September 11 terrorist attacks.

Mr Morsi, who was making his first visit to the West, also harshly criticised the film.

"We condemn strongly ... all those who launch such provocations and who stand behind that hatred," Mr Morsi said, adding that he had asked Mr Obama "to put an end to such behaviour".

Today, angry protesters stormed the US Embassy compound in Yemen's capital, Sanaa, tearing down and burning the US flag, in a scene reminiscent of the Cairo incident.

During a joint press conference, European Commission head Jose Manuel Barroso said the EU is prepared to make available 500 million euro in financial assistance to Egypt to support the consolidation of democracy.

Mr Morsi, who is hoping to carry out structural reforms to overhaul Egypt's ailing economy, is seeking a 4.8 billion dollar loan from the International Monetary Fund, but more could be required.

The two men also urged Syrian President Bashar Assad to step down to end the escalating civil war in that country.

"We are also adamant that Assad should go," Mr Barroso said. "We need a transition to an inclusive democracy."

Mr Morsi interrupted to add that this was "completely agreed upon."