The United States warned its citizens on Thursday not to travel to Egypt and called on those already there to leave.

The travel warning was issued one day after a brutal crackdown on street protests in Cairo ordered by Egypt's military-backed government left more than 500 people dead.

Senior U.N. human rights officials warned Thursday that Egypt's Christian minority faces reprisal attacks after the army's brutal crackdown on supporters of ousted president Mohammed Morsi.

Coptic Christians make up between six and ten percent of Egypt's population and their community has been the target of sectarian violence in the past.

The United States will retain its military ties with Egypt but more violence by the army could jeopardize the relationship, Defense Secretary Chuck Hagel told the country's military chief Thursday.

Hagel said he had called General Abdel Fattah al-Sisi, Egypt's defense minister and the central figure in the interim government, to express US concern after Wednesday's brutal crackdown on supporters of ousted president Mohammed Morsi.

The further the black-clad Egyptian policemen tightened the noose on the Rabaa al-Adawiya protest camp, the more desperate became the search for a place to lay out the protesters felled by their gunfire.

Amid a swarm of hissing bullets, two protesters barged into the garden of the Rabaa al-Adawiya mosque compound in east Cairo, carrying a man whose face was masked in blood.

Egypt's Tamarod group, which launched the call for mass protests that led to the ousting of Islamist president Mohammed Morsi, urged all Egyptians to take to the streets on Friday to defend the country from "terrorism".

"During these difficult times, we must all stand together... to defend the future of our children from terrorism and the dark forces which want to drag us back centuries," the group said.

The U.N. Security Council will hold an emergency meeting on Egypt at 2130 GMT on Thursday, following a request from France, Britain and Australia, diplomats said.

The meeting comes a day after a crackdown by Egyptian security forces on protesters backing the deposed Islamist president Mohammed Morsi, which left more than 500 people dead.

Egypt faces new polarization and the potential for further violence after a crackdown on supporters of ousted president Mohammed Morsi turned into a bloodbath, analysts say.

More than 500 people were killed on Wednesday when security forces moved to disperse two Cairo protest camps of Morsi loyalists in a heavy-handed assault that drew international censure.

U.S. President Barack Obama said Thursday the United States has canceled military exercises with Egypt to protest the killing of hundreds of protesters, in his first public statement on the rapid developments in Egypt.

He interrupted his weeklong vacation to address the clashes that have left more than 500 people dead.

Umm Abdallah sat silently praying over the body of her husband, killed in a bloody police crackdown on supporters of ousted Islamist president Mohammed Morsi, before collapsing into broken sobs.

The corpse wrapped in a white shroud with his name "Mohsen Radi" handwritten in thick black marker lay on the ground of the makeshift morgue at the Iman mosque, a few kilometers from the Rabaa al-Adawiya protest site where over 200 people were killed.

Tunisia's political crisis showed no sign of being resolved on Thursday, amid warnings against a slide into the carnage that has swept fellow Arab Spring country Egypt since democratic polls.

Rached Ghannouchi, head of the moderate Islamist party Ennahda that heads a coalition cabinet, slammed the opposition as "anarchists" for demanding the resignation of Tunisia's elected government.