A rocket was fired from south Lebanon towards Israel, Lebanese security sources said, and residents of a northern Israeli town reported hearing a blast.
Israeli forces and UN troops investigated the area near the Lebanese border early on Monday but did not detect any activity or damage.
"An explosion was heard. Soldiers are searching the area. The cause is still being investigated," an Israeli military spokeswoman said.
A second Israeli military source said the explosion was probably caused by a mortar.
The incident came amid heightened tensions in the region over Syria's conflict.
Damascus has said it will respond to Israeli air strikes earlier this month against suspected Iranian missiles in Syria destined for the Lebanese group Hezbollah.
The rocket launch could be heard from the Lebanese town of Marjayoun, about 10km from the Israeli border, residents in the Lebanese town said.
Earlier on Sunday, two rockets were fired into a Shia district of southern Beirut after Hezbollah leader Hassan Nasrallah pledged his Shia Muslim group would fight in Syria until victory for President Bashar al-Assad.
Assad is battling a two-year rebellion in which the United Nations says at least 80,000 people have been killed.