"Sources close to Hezbollah say response to Israeli strike is 'inevitable', response will be 'unexpected & painful,'" Lebanon's As-Safir daily said in a Twitter post.
The threat came after the Israeli army carried out an airstrike against militant forces close to the Israeli border, allegedly involved in a reconnaissance mission, several news outlets reported Sunday.
According to news channel Al-Arabiya, the attack killed the son of a late military commander, and Hezbollah field commander Mohammed Issa, in charge of the group's operations in Syria and Iraq.
Hezbollah, a paramilitary and political organization with roots in Lebanon's Shiite population, was established in the 1980s, engaging in military opposition to Israel's occupation of Lebanon.
Hezbollah militants have allegedly been taking part in the Syrian government's operations against the armed opposition and the Islamic State (IS).