An Israeli strike on southern Lebanon early on Monday killed a field commander in the heavily-armed Iran-backed Lebanese group Hezbollah, as the United Nations warned that shelling was spreading.

Hezbollah and the Israeli military have been exchanging fire across Lebanon's southern frontier in parallel with the Gaza war, adding to fears of a wider regional conflict.

Early on Monday, Israeli fighter jets hit the village of al-Sultaniyah and killed a field commander in Hezbollah's elite Radwan units and two other people, the Israeli military and two Lebanese security sources said.

The Israeli military identified the commander as Ali Ahmed Hassin, and said he was responsible for planning and executing attacks against Israelis. Hezbollah issued a funeral notice for Hassin but without details of his role.

Israeli strikes have killed around 270 Hezbollah fighters in the last six months as well as around 50 civilians, including children, medics and journalists. Hezbollah's rocket fire has killed around a dozen Israeli soldiers and half as many civilians.

In a joint statement on Monday, United Nations' Special Coordinator for Lebanon Joanna Wronecka and the commander of the UN peacekeeping mission in Lebanon, Aroldo Lazaro, said the violence must stop.

"The unrelenting cycle of strikes and counterstrikes in breach of the cessation of hostilities constitute the most serious violation of Security Council Resolution 1701 since its adoption in 2006," they said.

That UN decision ended a month-long war between Hezbollah and Israel nearly two decades ago but many of its points - including a withdrawal of armed groups from the south and deployment of Lebanese army troops - were never implemented.