Washington DC - Israel threatened to send Lebanon "back to the Stone Age" as the UN's humanitarian chief warned that a full-blown war between the countries would be "potentially apocalyptic."

The border between the two countries has seen daily exchanges of fire between Israeli forces and Iran-backed Hezbollah militants ever since the October 7 attack on Israel led by Hezbollah ally Hamas, which triggered the war in Gaza.

Fears those exchanges could escalate into full-blown war have only grown in recent weeks as cross-border attacks intensified, and after Israel revealed it had approved plans for a Lebanon offensive.

Israeli Defense Minister Yoav Gallant, who faces an International Criminal Court arrest warrant over alleged war crimes committed in Gaza, said during a visit to Washington on Wednesday that his country could "take Lebanon back to the Stone Age, but we don't want to do it".

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"We do not want war, but we are preparing for every scenario," he told reporters. "Hezbollah understands very well that we can inflict massive damage in Lebanon if a war is launched."

Though the US has said it is focused on de-escalating, it has also promised to back Israel no matter what – as has been the case with the brutal Israeli assault on Gaza, which has killed well over 37,000 Palestinians and continues to roil the entire region.

US Defense Secretary Lloyd Austin told Gallant on Tuesday that another war with Hezbollah could have "terrible consequences for the Middle East", and urged a diplomatic solution.

UN humanitarian coordinator Martin Griffiths told reporters in Geneva on Wednesday that Lebanon was "the flashpoint beyond all flashpoints".

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"It's beyond planning. It's potentially apocalyptic," warned Griffiths, whose term ends this week.

A war involving Lebanon "will draw in Syria... it will draw in others," he added. "It's very alarming."