Islamic State militants clashed with Syrian Al Qaeda-affiliate Nusra Front on Wednesday in the Lebanese region of Arsal, bordering Syria, a military official told Efe news agency.
The Lebanese army is on alert in the area, which has been the scene of frequent attacks and clashes with Islamist militias.
Across the Syrian border, in al-Qalamoun, the Lebanese Shia militant group Hezbollah and the Syrian regime have been jointly combating Syrian rebel militias and Islamist brigades.
However, the official denied accusations that Lebanese armed forces have been involved in any fighting in al-Qalamoun.
On Tuesday, several Syrian rebel factions declared their intention to eradicate the IS, which they have branded as a "corrupt" organisation.
Fighting in the border area has picked up in recent days, while the Syrian regime and Hezbollah have reportedly made progress against their adversaries.
