"Lebanon is becoming a key flashpoint," UN rights chief Volker Turk told reporters in Geneva.

"I call for an immediate cessation of hostilities."

Lebanon has been engulfed by the expanding Middle East war, after the Iran-backed group Hezbollah on Monday fired missiles at Israel to avenge the death of Iranian supreme leader Ali Khamenei.

Israel responded with waves of air strikes, and Thursday night it escalated its response by hitting Beirut's southern suburbs where Hezbollah is active -- after warning the area's hundreds of thousands of residents to flee.

Lebanese Prime Minister Nawaf Salam has warned that "a humanitarian disaster is looming" due to the mass displacement.

Turk said he was particularly worried about Israel's "blanket, massive displacement orders" for Beirut's southern suburbs, the Bekaa region and the full area to the south of the Litani river.

These orders were impacting "hundreds of thousands of people", he said, raising "serious concern under international humanitarian law and in particular when it comes to issues around forced transfer".

Turk's spokeswoman Ravina Shamdasani cautioned that the orders risked amounting to "prohibited forced displacement" under international law.

The mass displacement, coupled with "continued air strikes on different parts of the country, are bringing more misery and suffering to an already weary civilian population", she told reporters.

The Israeli military announced Friday that it had carried out 26 waves of strikes on Beirut's southern suburbs in the past four days.

Late Thursday, Lebanon's health ministry said Israeli strikes since Monday had killed 123 people.

Shamdasani pointed to reports that at least eight people were killed in a strike on a residential building in Baalbek on Wednesday, including three girls and two women, and a family of four reportedly died when a building was struck in the Nabatyeh district on Thursday.

"Prompt and thorough investigations must be conducted, particularly to determine whether such attacks complied with the principles of distinction, proportionality and precaution," she said.

Shamdasani highlighted that "Hezbollah has continued launching barrages of rockets into Israel, striking residential areas in the north and the centre of the country, with at least three people reportedly injured".

This, she said, raised "concerns, once again, about indiscriminate attacks against civilians".

She called for "urgent de-escalation", insisting "the sovereignty of Lebanon and the human rights of its people must be respected", she said.