BEIRUT, Lebanon — Two rockets crashed into southern Beirut suburbs controlled by the militant Shiite group Hezbollah on Sunday, wounding four people. The attack, the first on the group’s Beirut stronghold since the hostilities in Syria broke out two years ago, raised anxieties here that the fighting next door was beginning to revive Lebanon’s own sectarian conflicts.

Hezbollah’s leader, Hassan Nasrallah, declared Saturday in the strongest terms yet that the group had become a major combatant in Syria, taking the side of President Bashar al-Assad and vowing to fight to the end to defeat the rebellion and defend Lebanon and the region from jihadist extremists.

Some Hezbollah supporters said Sunday that they suspected Syrian rebels, who are mainly Sunni Muslims, or the Lebanese Sunni militants who support them, of mounting the rocket attack. A Hezbollah official said the attackers were part of a single “chain of terrorism” forged by Israel and stretching from Baghdad through Syria to Beirut.

But it was unclear who launched the rockets, which the Lebanese authorities said were fired from a primarily Christian and Druze area in the hills southeast of the city. No one was killed and no group immediately claimed responsibility.