BEIRUT, Lebanon — Syrian rebels battled fighters from the Lebanese militant group Hezbollah early on Sunday in a border region between the two countries, a Lebanese security official said, leaving as many as 12 fighters dead in the latest sign that Lebanon is being pulled into Syria’s lengthy civil war.

The security official, who spoke on the condition of anonymity because he was not authorized to talk to the news media, and local news reports said that the clashes occurred on Lebanese soil. If confirmed, they would represent some of the worst direct spillover fighting in Lebanon since Syria’s uprising began more than two years ago.

The clashes, which lasted for hours, started when Hezbollah fighters tried to stop a group of Syrian rebels who were setting up “rocket launchers” near the Lebanese city of Baalbek in the Bekaa Valley, according to the security official.

Fears for Lebanon’s security have simmered since the beginning of Syria’s uprising, as refugees flooded in and the fighting upended Lebanon’s precarious politics. Sunday’s violence crystallized Lebanon’s growing predicament: as Hezbollah has deepened its involvement in the war, fighting alongside the Syrian Army against the rebels, opponents of President Bashar al-Assad of Syria appear to be making good on threats to retaliate, further destabilizing the country.