Israel killed militant group Hezbollah's top military official in an airstrike on a southern suburb of Lebanese capital Beirut on Sunday, the Israeli military said, despite a US-brokered truce a year ago, Reuters reports.

The strike, the first on the outskirts of the Lebanese capital in months, targeted Iran-backed Hezbollah's acting chief of staff, Ali Tabtabai, the military said in a statement.

Hezbollah confirmed the death of Tabtabai in a statement.

Israel's strike crossed a "red line", Hezbollah official Mahmoud Qmati said as he stood near the bombed-out building in the Haret Hreik suburb, a Hezbollah stronghold.

Hezbollah's leadership would decide on whether and how the group would respond, he added.

The United States imposed sanctions on Tabtabai in 2016, identifying him as a key Hezbollah leader and offering a reward of up to $5 million for information on him.

Lebanon's health ministry said the strike killed five people and wounded 28 more. It hit a multi-storey building, sending debris crashing into cars on the main road below.

People rushed out of their apartment buildings, fearing further bombardment, a Reuters reporter said.

Lebanese President Joseph Aoun urged the international community to intervene to halt Israeli attacks.