U.S. senators John McCain and Lindsey Graham are due to arrive in Cairo as diplomatic efforts to end Egypt's political crisis intensify, BBC News reports.

U.S. and EU envoys have been meeting officials from the military-backed government and supporters of ousted President Mohammed Morsi.

Their talks took place amid mounting tension over plans to break up two mass sit-ins by Morsi loyalists in Cairo.

A government helicopter dropped leaflets urging people to leave. The pamphlets, which were dropped over the Rabaa al-Adawiya square, pledged protesters would face no action if they had not committed a crime whilst demonstrating.

More than 100 Morsi supporters have been killed in clashes since the president was overthrown by the military on July 3.

Meanwhile a court in Cairo said it would start the trial of Muslim Brotherhood leader Mohammed Badie and two of his deputies on August 25.

McCain and Graham are expected to land in the Egyptian capital on Monday, Aug 5 evening for two days of negotiations. Over the weekend, U.S. deputy secretary of state, William Burns, already held separate talks with members of Morsi's Muslim Brotherhood and the Egyptian Foreign Minister, Nabil Fahmy.

The European Union's envoy, Bernardino Leon, also joined the meeting with Fahmy.

The foreign ministers of Qatar and the United Arab Emirates are also involved in the diplomatic push, and are reported to have visited a Muslim Brotherhood leader.