There were no reports of injuries in Thursday's blasts, but the Israeli army said they were presumed to be rocket attacks.
There was no immediate claim of responsibility for the blasts, which came a day after a rocket was fired from south Lebanon that landed in a northern Israeli town, damaging a building but causing no casualties.
"These are presumed attacks from Lebanese territory," an Israeli army spokeswoman said.
She said Israel had dispatched army helicopters to the area, but soldiers would not be able to conduct a thorough probe until morning.
A spokeswoman for UN Secretary-General Kofi Annan said he was gravely concerned after Wednesday's rocket attack.
"The secretary-general urges all parties to fully respect the Blue Line and reminds them that one violation of the Blue Line cannot justify another," she said, referring to the UN-drawn frontier marking Israel's withdrawal from south Lebanon.
No group had claimed responsibility for Wednesday's rocket fire.
Palestinian resistance fighters have launched rocket attacks on Israel from across the border in recent years.