Lebanese militant group Hezbollah fired rockets and mortars at what it said were ISIS fighters in the north of the country on Friday, killing five of them, a security source said.
Sunni Islamists have regularly infiltrated Lebanon's border with Syria in the north during its neighbour's four-year-old civil war, clashing with both Shia Hezbollah and the Lebanese army.
The Shia movement's al-Manar TV also reported the incident, which took place in the northern Bekaa Valley. It was not followed by further violence, the security source said.
ISIS, which holds vast areas of territory in eastern Syria, has a much smaller presence in the west of the country and near the Lebanese border. Other Islamist groups including al Qaeda's Syrian branch Nusra Front are more active in western Syria.
Syria's civil war has spilled over into Lebanon, which is still rebuilding after its own 15-year civil war. There have been clashes between gunmen loyal to opposing sides in the Syrian conflict, as well as strikes on the army and cross-border attacks by Syrian rebels.
The army fought several days of deadly battles last year with insurgent groups including ISIS and Nusra Front when they staged an incursion into the town of Arsal near the more than 300 km (190 mile) border with Syria.
The powerful Shia Hezbollah is fighting on the side of President Bashar al-Assad against Sunni insurgents inside Syria.