Tuesday’s threat to bring the fight to Lebanon, issued by the Free Syrian Army, came after several days of clashes between Syrian rebels and militants from Hezbollah around Qusayr, a Syrian town near the Lebanese border, and a handful of smaller villages in the area, according to the rebels. Hezbollah has not officially responded to the Free Syrian Army statement.

Hezbollah is the most powerful military and political force in Lebanon, and it runs an extensive network of social programs. Last fall, Hezbollah leader Hasan Nasrallah acknowledged that some of the organization’s military personnel were inside Syria, but said they were not involved in combat.

A rebel attack on Hezbollah targets inside Lebanon could have broad regional implications. Hezbollah is closely allied with Iran, which is one of Assad’s strongest backers. The Syrian rebels, on the other hand, have largely been supported by Turkey, Qatar and Saudi Arabia.

There also is a potentially explosive sectarian dynamic as Hezbollah and Iran are both predominantly Shiite Muslim while most members of the Syrian opposition — and the bulk of their foreign backers — are Sunni Muslim.