The European Union's foreign policy chief was scheduled to hold crisis talks in Cairo on Monday, July 29 after the weekend killing of at least 72 supporters of Egypt's deposed Islamist president plunged the pivotal Arab country deeper into turmoil, Reuters reported.

Underscoring the risk of more bloodshed, several thousand supporters of ousted President Mohamed Morsi's Muslim Brotherhood threatened to march on the military's intelligence headquarters in defiance of a warning from the army to stay away.

They turned back early Monday, having left the site of a Brotherhood vigil in northern Cairo chanting, "Our blood and souls we sacrifice for Morsi."

The dawn killings on Saturday, following a day of rival mass rallies, deepened the turmoil plaguing the country since the army shunted Egypt's first freely elected president from power on July 3.

The West is increasingly concerned about the risk of broader conflict in the Arab world's most populous country, a bridge between the Middle East and Africa and recipient of more than $1 billion in military aid from the United States.

Catherine Ashton, the EU's foreign policy chief, was scheduled to meet General Abdel Fattah al-Sisi, the head of the Egyptian armed forces who led the overthrow of Mursi, the country's interim president, Adli Mansour, and officials of the Freedom and Justice Party, the Brotherhood's political wing.

Ashton, in a statement, said she would press for a "fully inclusive transition process, taking in all political groups, including the Muslim Brotherhood."