National News said early Monday the reasons behind the blast near Tair Harfa were not clear.
Andrea Tenente, who is a spokesman for the U.N. peacekeeping force in the area, UNIFIL, said it was investigating.
South Lebanon, the scene of bitter fighting between Israel and Lebanese militant Hezbollah guerrillas in 2006, is considered a Hezbollah stronghold.
Security officials said Hezbollah had cordoned off the area. They asked not to be named because they are not authorized to speak publicly.