Hezbollah, the Lebanese ally of Gaza's Hamas, said it had hit a key Israeli observation post with 62 rockets.
That was described as a "preliminary response" to the killing of Hamas' deputy chief Saleh al-Arouri on Tuesday (January 2).
Israel's military said "approximately 40" launches had been identified, from Lebanon towards the area of Meron in northern Israel.
It said it had responded with an unmanned aerial vehicle strike on, quote, "the terrorist cell responsible".
According to the military, Israeli fighter jets and troops also struck a series of Hezbollah targets in southern Lebanon.
Arouri was killed in an attack widely attributed to Israel.
The head of Hezbollah, Hassan Nasrallah, said on Friday (January 5) that Lebanon would be "exposed" to more Israeli operations if his group did not respond to the killing.
Concerns over escalating regional tensions prompted a renewed diplomatic push on Saturday.
Israel and Hezbollah have often traded fire across the Lebanese border; the West Bank is seething with emotion; and the Iran-aligned Houthis in Yemen seem determined to continue attacks on Red Sea shipping lanes until Israel stops bombarding Gaza.
The European Union's foreign policy chief, Josep Borrell, was in Lebanon and had this to say.
"It is imperative to avoid a regional escalation in the Middle East. It's absolutely necessary to avoid Lebanon being dragged into a regional conflict."
He added that his message to Israel was that nobody would win from a regional conflict.
U.S. Secretary of State Antony Blinken arrived in Turkey at the start of a week-long regional tour that also takes in Israel, the occupied West Bank, Jordan, Qatar, the United Arab Emirates, Saudi Arabia and Egypt.
A State Department spokesperson said Blinken had emphasized to Turkish President Tayyip Erdogan the need to "prevent the conflict from spreading".
