UNITED NATIONS, United States, Sept 24, 2024 (BSS/AFP) - Escalating clashes between Israel and Hezbollah threatened to overshadow US President Joe Biden's final appearance at the UN's signature annual event on Tuesday as diplomats scrambled to avert an all-out regional war.

The gathering of dozens of world leaders, the high point of the diplomatic calendar, comes a day after Israeli air strikes on Lebanon killed more than 490 people, according to local authorities.

As world leaders gathered in Manhattan Monday for the annual flurry of speeches and face-to-face diplomacy, UN Security Council member France called for an emergency meeting on the crisis engulfing the Middle East.

UN Secretary-General Antonio Guterres's spokesman said he was "gravely alarmed" as focus shifted from Gaza to Lebanon, and the EU's top diplomat Josep Borrell warned "we are almost in a full-fledged war."

Israel's closest ally the United States again warned against a full-blown ground invasion of Lebanon, with a senior US official promising to bring "concrete" ideas for de-escalation to the UN this week.

It is unclear what progress can be made to defuse the situation in Lebanon as efforts to broker a ceasefire in Gaza, which Israel has relentlessly pounded since October 2023, have come to nothing.

"Gaza, Ukraine and Sudan will be the dominant issues," said Richard Gowan of the International Crisis Group think tank, adding he expected many leaders to "warn that the UN will become irrelevant globally if it cannot help make peace."

More than 100 heads of state and government are scheduled to speak during the UN's centerpiece event, which will run until Monday.

- 'Out-of-control' -

Since last year's annual gathering, when Sudan's civil war and Russia's Ukraine invasion dominated, the world has faced an explosion of crises.

"International challenges are moving faster than our ability to solve them," Guterres warned ahead of the gathering.

The October 7 attack by Palestinian Islamist group Hamas on Israel and the ensuing violence in the Middle East has exposed deep divisions in the global body.

With Israel's leader Benjamin Netanyahu and Palestinian president Mahmud Abbas expected to address the General Assembly this week, there could be combustible moments.

On Tuesday, representatives of Turkey, Jordan, Qatar, Iran and Algeria are slated to take the podium to press for a Gaza ceasefire after nearly one year of war.

Ukraine will also be on the agenda Tuesday when President Volodymyr Zelensky addresses a UN Security Council meeting on Russia's war on Ukraine.

"I invite all leaders and nations to continue supporting our joint efforts for a just and peaceful future," Zelensky told the UN on Monday.

"Putin has stolen much already, but he will never steal the world's future."

- 'Behind the scenes' -

It is unclear if the grand diplomatic gathering can achieve anything for the millions mired in conflict and poverty globally.

"Any real diplomacy to reduce tensions will take place behind the scenes," Gowan said.

"This may be an opportunity for Western and Arab diplomats to have some quiet conversations with the Iranians about the need to stop the regional situation spinning out of control."

Iraqi Prime Minister Mohammed Shia al-Sudani has called for an urgent meeting of Arab leaders on the sidelines of the UN General Assembly over the crisis in Lebanon.

Guterres cautioned against "the possibility of transforming Lebanon (into) another Gaza."