CAIRO — An Islamic summit that opened in Egypt on Wednesday lay bare the multiple divisions within the Muslim and Arab worlds, with conflicting approaches to the Syrian civil war exposing the Sunni-Shi’ite sectarian fault lines that have torn the region for years.

Egypt’s Islamist leader sharply criticized President Bashar Assad’s embattled regime in his address to the two-day summit, though he hedged his comments by only making an indirect call for the Syrian leader to step down.