BEIRUT, Lebanon — The deadly clashes that raged between rival rebel factions in Syria over the weekend accentuated the divisions hampering opponents of President Bashar al-Assad as they try to halt his forces’ recent gains on the battlefield and persuade the West to supply the insurgency with weapons.

The fighting flared between members of a mainstream rebel group and a radical faction affiliated with Al Qaeda, according to local residents and an antigovernment watchdog group. The presence of the radicals and the failure to bring rebel forces under a unified military leadership have made the United States and its allies reluctant to arm the opposition.

As foreign fighters continued to spill into Syria across the country’s porous borders — committing atrocities against both supporters and opponents of the government and clashing with more moderate rebel groups — the prospects for unity among the rebels have seemed to grow more remote.

Islamist fighters said recently that they had driven a rival rebel brigade out of Raqqa, a rebel-held provincial capital in northeastern Syria, because they had found some of its fighters drinking wine and consorting with women, and because they considered brigade reluctant to fight.