Lebanese militant group Hezbollah fired rockets and mortars at what it said were Islamic State 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 neighbor's four-year-old civil war, clashing with both Shi'ite Hezbollah and the Lebanese army.
The Shi'ite 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.
Islamic State, 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 Islamic State 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 Shi'ite Hezbollah is fighting on the side of President Bashar al-Assad against Sunni insurgents inside Syria.
(Reporting by John Davison; Editing by Toby Chopra)