Shooting of pro-Mursi protesters deepens Egypt crisis By Tom Perry and Maggie Flick CAIRO (Reuters) - At least 15 people were killed in Cairo on Monday, medical sources said, when the Muslim Brotherhood said shots were fired at supporters of deposed Egyptian President Mohamed Mursi near the military building where he is being held. The bloodshed deepened Egypt's political crisis, escalating the struggle between the army, which overthrew Mursi last Wednesday after mass demonstrations demanding his resignation, and the Brotherhood, which has denounced what it called a coup. The military said "a terrorist group" tried to storm the Republican Guard compound and one army officer had been killed and 40 wounded. Soldiers returned fire when they were attacked by armed assailants, a military source said. The Brotherhood's official spokesman, Gehad El-Haddad, who is at a pro-Mursi sit-in at a mosque near the scene, said 37 Mursi supporters had been killed. He said shooting broke out in the early morning while Islamists were praying and staging a peaceful sit-in outside the Republican Guard barracks. "We call on all patriotic brave Egyptians 2 join us @... sitin to defend country from conspiratorial traitors of military coup," he said in a Twitter message. As an immediate consequence, the ultra-conservative Islamist Nour party, which initially supported the military intervention, said it was withdrawing from stalled negotiations to form an interim government for the transition to fresh elections. Al Jazeera's Egypt news channel broadcast footage of what appeared to be five men killed in the violence, and medics applying cardiopulmonary resuscitation to an unconscious man at a makeshift clinic at a nearby pro-Mursi sit-in. A Reuters television producer at the scene saw first aid helpers attempting mouth-to-mouth resuscitation on a dying man. Wounded people were being ferried to the field hospital on motorbikes, given first aid treatment and taken away in ambulances. The military overthrew Mursi on Wednesday after mass nationwide demonstrations led by youth activists demanding his resignation. The Brotherhood denounced the intervention as a coup and vowed peaceful resistance. Military vehicles sealed off traffic in a wide area around the Rabaa Adawia mosque where Mursi supporters led by senior Brotherhood leaders have been staging protests since his ouster. The army also closed two of the main bridges across the Nile River with armored vehicles, witnesses said. Talks on forming a new government were already in trouble before Monday's shooting, after the Nour party rejected two liberal-minded candidates for prime minister proposed by interim head of state Adli Mansour. Nour, Egypt's second biggest Islamist party, which is vital to give the new authorities a veneer of Islamist backing, said it had withdrawn from the negotiations in protest at what it called the "massacre at the Republican Guard (compound) ." "We've announced our withdrawal from all tracks of negotiations as a first response," party spokesman Nader Bakar said on Facebook. The military can ill afford a lengthy political vacuum at a time of violent upheaval and economic stagnation in the Arab world's largest nation of 84 million people. The army appeared to be counting on exhaustion and the onset of the Muslim fasting month of Ramadan from Tuesday to wear down the Brotherhood protesters. However, even before Monday's incident, many were determined to hold out and die for their cause if necessary. Hanim Ahmad Ali Al-Sawi, 55, spent Sunday encamped outside the Republican Guard barracks where Mursi has been helped since the coup. People blamed the Brotherhood for economic stagnation and said it was trying to take over every part of the state, an accusation the movement stringently denies. Additional reporting by Ashraf Fahim, Asma Alsharif, Mike Collett-White, Alexander Dziadosz, Maggie Fick, Tom Finn, Sarah McFarlane, Tom; Perry, Yasmine Saleh, Paul Taylor and Patrick Werr in Cairo and Steve Holland and Patrick Temple-West in Washington; Writing by Mike Collett-White and Paul Taylor; Editing by Philippa Fletcher