Senior U.S. diplomat Bill Burns on Monday urged Egypt's divided factions to engage in dialogue and end violence, two weeks after the military ousted the country's first freely elected president.

"The first priority must be to end violence and incitement, prevent retribution, and begin a serious and substantive dialogue among all sides and all political parties," Burns said.

U.N. Secretary General Ban Ki-moon warned Monday against "revenge" and "retribution" in Egypt, which has been hit by large-scale protests since President Mohammed Morsi was overthrown.

The country's first freely elected president was deposed on July 3, and a military-installed caretaker government has since tightened the screws on the Islamist leader's backers.

Egypt's Christians have been targeted in a wave of attacks since the ouster of Islamist president Mohammed Morsi, and the state is failing to protect them, an NGO said Monday.

Sectarian violence since the latest political upheaval in Egypt began has killed four Coptic Christians in Luxor governorate, with churches elsewhere torched and looted, said the Egyptian Initiative for Personal Rights.

Egyptian officials turn back a planeload of Syrians at Cairo airport. A popular presenter on Egyptian television warns Syrians to steer clear of protests or face the consequences. An Egyptian state school refuses admission to Syrian children.

Once welcomed with open arms in Egypt, many of the tens of thousands of Syrians who took refuge in Egypt from the civil war at home have now found themselves targets of hate speech and intimidation. Their dramatic change in fortune is one of the unexpected consequences of the Egyptian military's ouster of President Mohammed Morsi, whose Islamist-dominated government offered them favorable conditions.

Militants killed at least three people Monday and wounded 17 when they fired on a bus carrying workers in the North Sinai town of Al-Arish, security and medical sources said.

Military spokesman Colonel Ahmed Aly said in a statement that a "terrorist group" had been targeting a police vehicle but hit the workers' bus by mistake.

Gunmen clashed with the Egyptian army on Sunday near the Israeli border, security sources said, in the latest violence to erupt in the Sinai peninsula since the ouster of president Mohammed Morsi.

The fighting broke out in the area of al-Wifaq, in northern Sinai, after militants tried unsuccessfully to blow up a police vehicle with explosives.

Diehard supporters of Egypt's deposed president Mohamed Morsi have been rallying in Cairo for nearly two weeks calling for his reinstatement, but their prolonged protest has left them isolated.

Since they began the sit-in protest, their main source of news has been from speeches delivered from a podium set up by the Muslim Brotherhood, the group from which ousted Islamist leader Morsi hails.

German Chancellor Angela Merkel on Sunday called for the release of Egypt's former president Mohammed Morsi, who was arrested on July 3 after being deposed by the army.

She said she agreed with her Foreign Affairs Minister Guido Westerwelle "that Mr. Morsi should be released" and called for "an inclusive (political) process involving all groups of the Egyptian population", speaking with public broadcaster ARD.

Egypt's public prosecutor on Sunday ordered the freezing of assets belonging to 14 top Islamists as part of an investigation into deadly violence, judicial sources said.

The order by Hisham Barakat affects nine Muslim Brotherhood leaders, including the group's general guide Mohamed Badie, and five Islamists from other groups including ex-militant faction Gamaa Islamiya, the sources said.

A top U.S. official was heading to Cairo on Sunday for talks with interim government leaders, the first high-ranking administration member to visit Egypt since the ouster of Mohamed Morsi, as the EU voiced "deep concern" over the situation in the country.

Under Secretary of State Bill Burns will visit Egypt from Sunday to Tuesday, the State Department said, adding that he would "underscore U.S. support for the Egyptian people."