Hezbollah fired at least 20 rockets at Israeli-occupied Shebaa Farms in south Lebanon Friday in response to two Israeli airstrikes a day earlier.
The UN peacekeeping force, UNIFIL, said it detected rocket launches from Lebanon and return artillery fire by Israel.
"This is a very serious situation and we urge all parties to cease fire," it said in a brief statement.
A Hezbollah statement said the group fired dozens of 122 mm rockets at "open ground" in retaliation for two Israeli airstrikes on open ground in south Lebanon early Thursday.
"The Islamic Resistance rocketed open ground near positions of Israeli occupation forces in the Shebaa Farms with tens of rockets," it said.
Israeli artillery positions responded to Friday's salvo by firing 155 mm rounds into the outskirts of Habbariyeh village near Shebaa as Israeli warplanes flew overhead.
Sirens warning of incoming rocket fire were sounded in northern Israel and the Israeli-occupied Golan Heights, the Israeli military said. There were no immediate reports of damage or casualties. Israeli media reported that some of the rockets had been intercepted by Israel's Iron Dome defense system.
Some angry residents of Shwayya village in the area intercepted a truck with a rocket launcher as it drove through after the rockets were fired, a security source said. Lebanese Army soldiers detained four men and confiscated the truck, the source said.
Hezbollah, in a second statement, confirmed that its men and truck had been intercepted by villagers after firing the rockets from a nearby location. It said it is always careful not to endanger the safety of residents.
Thursday's airstrikes on south Lebanon were Israel's first in 15 years. Two rockets hit northern Israel Wednesday, drawing Israeli artillery fire.
Lebanon and Syria say Shebaa Farms are Lebanese while Israel says they are part of the Golan Heights, occupied by Israel in 1967.
Source: The Daily Star