The outgoing United Nations humanitarian chief warned Wednesday that a spread of the Israel-Hamas war to Lebanon would be "potentially apocalyptic", as fighting raged in the southern Gaza Strip.

Martin Griffiths described Lebanon as "the flashpoint beyond all flashpoints", especially its southern border with Israel which has seen daily cross-border violence since the October 7 Hamas attack on Israel.

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

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

Germany on Wednesday echoed a Canadian warning from the day before, urging their citizens in Lebanon to leave the country.

"German nationals are urgently requested to leave Lebanon," updated foreign ministry advice in Berlin said.

"The current heightened tensions in the border area with Israel could escalate further at any time."

On Tuesday, Ottawa advised Canadians to leave Lebanon "while they can".

The UN's Griffiths spoke as witnesses reported intense fighting between Israeli troops and Palestinian militants in Gaza's southern city of Rafah, amid the fears of a wider war.

With the conflict nearing its 10th month, Israel's top ally the United States warned of the risk of a major conflict with Hezbollah following an escalation in threats after months of cross-border fire.

However, Israeli bombardment in Gaza appeared to ease days after Prime Minister Benjamin Netanyahu said the war's "intense phase" was nearing its end, and as his defence minister was in Washington.

"Another war between Israel and Hezbollah could easily become a regional war, with terrible consequences for the Middle East," US Defense Secretary Lloyd Austin told his visiting Israeli counterpart, Yoav Gallant.

"Diplomacy is by far the best way to prevent more escalation," Austin said.

Gallant, for his part, said: "We only fight those who seek to harm us."

He also said "significant progress" had been made in addressing Israeli concerns about the flow of US weaponry.

Netanyahu has publicly accused US President Joe Biden's administration of slowing down the weapons deliveries, claims officials in Washington have repeatedly denied.

Top Israeli officials, including the premier, have indicated openness to a diplomatic resolution of the Lebanon border tensions, though Gallant said Israel should be ready for "every possible scenario".

Israel's military said last week plans for a Lebanon offensive had been "approved and validated", prompting new threats from Hezbollah chief Hassan Nasrallah.

In Rafah, on Gaza's border with Egypt, witnesses reported overnight clashes, and Israel's military said warplanes struck a rocket launch site.

Mohammad al-Mughayyir, a civil defence official in Hamas-run Gaza, told AFP rescuers had recovered 15 bodies "from various areas in Rafah city".

- Aid group 'outraged' -

The civil defence agency and medics said at least four people, including three children, were killed in a strike on a house in Beit Lahia, in the north.

However, agency spokesman Mahmud Basal told AFP "there have been almost no attacks" and "the rest of the areas in the Gaza Strip are calm compared to yesterday".

An air raid on Tuesday killed Fadi al-Wadiya, an employee of medical charity Doctors Without Borders (MSF) whom Israel's military called a "significant operative" for the Islamic Jihad group which has fought alongside Hamas.

MSF posted on X that it was "outraged" by the killing in Gaza City of Wadiya and "five other people including three children" as he cycled to work.

Israel's military said Wadiya had "developed and advanced the terrorist organisation's rocket array".

UN and humanitarian agencies have repeatedly warned that aid workers are not safe in Gaza, impeding their desperately needed efforts delivering aid for Gaza's 2.4 million people.

In Cyprus, USAID officials said just 1,000 tonnes of the 7,000 tonnes of aid shipped to Gaza had been distributed because of looting and security problems.

- 'Rolling operations' -

The bloodiest ever Gaza war started with Hamas's October 7 attack on southern Israel that resulted in the deaths of 1,195 people, mostly civilians, according to an AFP tally based on Israeli figures.

The militants also seized about 250 hostages, 116 of whom remain in Gaza although the army says 42 are dead.

Israel's retaliatory offensive has killed at least 37,718 people, also mostly civilians, Hamas-run Gaza's health ministry said.

The deaths include 10 members of Qatar-based Hamas political chief Ismail Haniyeh's family, including his sister, who Palestinian officials said were killed on Tuesday.

UNRWA chief Philippe Lazzarini warned of the war's dire impact on children.

"We have every day 10 children who are losing one leg or two legs on average," Lazzarini told reporters, adding "that means around 2,000 children after the more than 260 days of this brutal war".

Netanyahu on Sunday said "the war in its intense phase is about to end in Rafah", which the Israeli military sees as Hamas's last stronghold, with some troops to be redeployed to the northern border with Lebanon.

Mairav Zonszein, an International Crisis Group analyst, said the military would likely "move to rolling operations" in Gaza and "always keep some troops on the ground" in strategic areas there.

© Agence France-Presse