The ultra-conservative Islamist Nour party, which backed the military overthrow of Egyptian President Mohamed Morsi , said Monday it had withdrawn from negotiations to form a new interim government in protest at what it called the "massacre of the Republican Guard."The statement came after the Muslim Brotherhood claimed that Egyptian security forces opened fire on a group of its supporters staging a sit-in outside where Morsi is believed to be held, killing 34 people."We've announced our withdrawal from all tracks of negotiations as a first response," Nader Bakar, spokesman for the Nour party, Egypt's second biggest Islamist party, said on Facebook.