The scale of the killing in Israeli strikes on Lebanon Wednesday is "horrific", the UN rights chief said, urging the international community to help end the unfolding "nightmare".

At least 112 people were killed and 837 more were wounded in Lebanon Wednesday after Israel launched a wave of strikes unprecedented in the current war, according to the latest Lebanese health ministry toll.

Volker Turk, the United Nations' High Commissioner for Human Rights, condemned the attacks, saying the numbers killed and injured were appalling.

"The scale of the killing and destruction in Lebanon today is nothing short of horrific," he said in a statement.

"Such carnage, within hours of agreeing to a ceasefire with Iran, defies belief. It places enormous pressure on a fragile peace, which is so desperately needed by civilians."

Mass casualties have been reported, and hospitals overwhelmed, the statement added.

A UN Human Rights team at the site of a strike in Beirut described a scene of devastation, with several dead bodies amid the rubble.

'Prompt investigations'

International humanitarian law spells out clearly that civilians and civilian infrastructure must be protected, said Turk.

"Each and every attack must comply with international humanitarian law fundamental principles of distinction, proportionality and precautions to protect civilians," he said.

"These principles are non-negotiable, and must always be respected, whatever the circumstances of armed conflict.

"There must be prompt and independent investigations into all alleged violations, and those responsible brought to justice."

Lebanon was drawn into the Middle East war on March 2 when Tehran-backed militant group Hezbollah began firing rockets into Israel to avenge the killing of Iranian supreme leader Ayatollah Ali Khamenei.

Turk also deplored Hezbollah's missile and drone attacks on northern Israel, and urged both parties to stop, immediately.

More than a million people have been displaced in Lebanon by the conflict, and Israel has launched a ground invasion in the south.

"The scale of such actions, coupled with statements by Israeli officials indicating an intention to occupy or even annex parts of southern Lebanon, are deeply troubling," said Turk.

"The international community must act quickly to help bring an end to this nightmare.

"Efforts to bring peace to the wider region will remain incomplete as long as the Lebanese people are living under continuing fire, forcibly displaced, and in fear of further attacks."