Two airstrikes targeted an area in the Sidon district on Thursday afternoon, Lebanese TV networks said.
The Israeli army said a warplane carried out an airstrike after "terrorist activity was detected at a Hezbollah facility containing medium-range rockets in south Lebanon."
"The IDF (Israeli army) is deployed in southern Lebanon, acting to thwart any violation of the ceasefire agreement," the Israeli military added.
This is the first such violation of the ceasefire that entered into effect on Wednesday morning.
An Israeli official told U.S. news portal Axios that Israel "notified the U.S. in advance" of the strike.
The mayor of the town of Baysariyeh in southern Lebanon, Nazih Eid, told AFP "a warplane launched a raid about an hour ago on the eastern edge of the town of Baysariyeh. They targeted a forested area not accessible to civilians."
The Israeli army earlier on Thursday said it fired at several suspects "arriving with vehicles to a number of areas in southern Lebanon, breaching the conditions of the ceasefire."