Three Syrians were injured in an accidental explosion at a munitions depot in the north Lebanon port of Tripoli on Friday, a security source told AFP.
The men, all security guards, lit a fire to warm themselves near containers that stored munitions, without being aware of the contents, prompting an explosion at the privately run facility.
The blast increased tensions in the city, where shooting broke out earlier as gunmen deployed heavily in two rival neighbourhoods, one group supporting and the other opposed to the regime in Syria.
"There is a heavy armed presence and shooting in the neighbourhoods of Bab al-Tebbaneh and Jabal Mohsen," a security official said, requesting anonymity.
He said the army had deployed in both neighbourhoods, but later retreated to a street dividing the two sides.
Five people were reported wounded, including two soldiers.
The coastal city has in the past few years been the scene of intense clashes between supporters of the anti-Syrian opposition and people loyal to a Hezbollah-led alliance.