Israeli troops regularly patrol the border area
Israeli officials said their troops came under fire as they searched a border area following the discovery of four explosives on Monday.
Israeli troops returned fire but no casualties were reported, security sources said.
Israel and Lebanon's Hezbollah militia fought a 34-day war last year sparked by flare-ups on the tense border.
Since that conflict the international border between Israel and Lebanon has been policed by a combination of Lebanese government soldiers and UN troops.
Hezbollah has denied the charge, saying the bombs had been there since last summer's fighting.The BBC's Simon Wilson, in Jerusalem, says the flare-up is serious, but not as serious as if the exchange of fire had involved Israel and Hezbollah fighters.