One soldier and a civilian were killed in the Lebanese city of Tripoli as clashes erupted between Sunni Muslims and members of the Alawite minority.
The violence began overnight as armed groups from an Alawite enclave clashed with Sunni fighters.
Tensions in the northern port city have mounted since the uprising in neighbouring Syria began last March.
Similar recent clashes have highlighted how tensions can spill over to Lebanon. Syria's president is an Alawite.
In February at least two people were killed in Tripoli as supporters and opponents of Syria's President Bashar al-Assad clashed.
But the city's Alawite minority has fought with its Sunni neighbours on several occasions since the 1975-1990 Lebanese civil war.
In the latest incident two rocket-propelled grenades fell on Tripoli's Bab Tabbaneh neighbourhood and reports say explosions were heard across the city.
Despite the deployment of Lebanese Army units between the rival neighbourhoods, exchanges of gunfire continued well into the morning.
Tripoli is dominated by Sunni Muslims, who support the anti-Assad uprising in Syria.
Mr Assad is an Alawite, and members of the minority sect - an offshoot of Shia Islam - occupy key positions in the Syrian government and security forces.
Syria's majority Sunni community has been at the forefront of the revolt against the president and borne the brunt of the state's crackdown.
The BBC's Jonathan Head in Istanbul says that community leaders in Lebanon have repeatedly warned of the possibility that the violence in Syria would spill over the border. Lebanon is already hosting thousands of Syrian refugees.
In recent years the fear of renewed civil war has helped persuade the various Lebanese factions to put aside their historic disagreements, but Syria is proving a very tough test for a country with so many sectarian divisions of its own, our correspondent says.