By Yasmine Saleh and Matt Robinson CAIRO (Reuters) - The United States urged Arab ally Egypt to pull "back from the brink" after security forces killed dozens of supporters of deposed Islamist President Mohamed Mursi and opened a dangerous new phase in the army's confrontation with his Muslim Brotherhood. Thousands of Brotherhood supporters were hunkered down in a vigil at a Cairo mosque on Sunday, vowing to stand their ground despite a threat by the authorities to disperse them "soon". ...