BEIRUT, Lebanon — A prominent Sunni Muslim cleric influential in the Syrian uprising has issued a fatwa, or religious decree, calling on Muslims around the world to help Syrian rebels in the embattled town of Qusayr and labeling Hezbollah and Iran, which support the Syrian government, enemies of Islam “more infidel than Jews and Christians.”

The cleric, Sheik Yusef al-Qaradawi, an Egyptian based in Qatar whom Syrian authorities have branded an extremist bent on inciting Syrian Sunnis but who is respected by many in the rebellion, made his statements in a sermon on Friday in Doha, Qatar, and posted them on his Web site. Though it was unclear how many would heed the call who were not already motivated to fight in Syria, his comments were bound to heighten sectarian passions in the region as the mostly Sunni rebels battle a government backed by Shiite Iran and Hezbollah.

Tensions also heightened on Saturday in Lebanon over the war in neighboring Syria. A mainstream Sunni Muslim organization called on Sunnis to join the fight against the Syrian government, and its leader was attacked as he drove through the Hezbollah-dominated Shiite area where he lives.

Sixteen shells and rockets fired from Syria on Friday night hit the outskirts of Baalbek, a city in Lebanon’s Bekaa Valley considered a stronghold of Hezbollah, the Shiite militant group that is also Lebanon’s most powerful political party. Around the same time, gunmen fired on a Shiite shrine there, according to Lebanon’s National News Agency.