United Nations Secretary-General Antonio Guterres warned world leaders gathering in New York that Lebanon was on the "brink," a day after 558 people were killed by Israeli strikes there.

"We should all be alarmed by the escalation. Lebanon is at the brink," Guterres said at the opening of the UN's signature annual event.

World leaders on Tuesday sounded the alarm over fears of a wider war after Lebanon suffered its deadliest day in two decades due to Israeli air strikes on Hezbollah strongholds.

After nearly a year of cross-border fire since the Gaza war erupted, Israeli bombardment on Monday killed 558 people in Lebanon, including 50 children, according to the country’s health ministry.

A Lebanese security source said an Israeli strike hit Hezbollah's south Beirut stronghold Tuesday, with the Israel army confirming it carried out a "targeted" strike in Beirut's southern suburbs.

At least six people were killed and 15 others wounded in the strike, the Health Ministry said.

Some 500 people have crossed from Lebanon to war-torn Syria, a Syrian security official told AFP Tuesday, fleeing the deadliest Israeli bombardment since Hezbollah and Israel fought a devastating war in 2006.

"Around 500 people crossed the border through the Qusayr and Dabousiya crossings between 4 pm (1300 GMT) and midnight," Monday, the security official told AFP, requesting anonymity to discuss sensitive matters. "Vehicles were still crossing in the early hours of the morning," he added.

Washington is conducting negotiations on the security developments between Lebanon and Israel on the sidelines of the U.N. General Assembly, sources have told Sky News Arabia.

A journalist working for the pan-Arab network Al-Mayadeen was killed in Israeli airstrikes while he was at his home in southern Lebanon, the network said Tuesday.

Hadi Al-Sayyed, 22, is the third journalist from the network killed in the ongoing conflict between the Israeli military and Lebanon's militant Hezbollah group. The network said he was wounded on Monday and died of his wounds on Tuesday.

The Kremlin said Tuesday that the escalation of the conflict between Israel and Lebanon's Hezbollah risked completely destabilizing the region.

"It is, of course, an event that is potentially very dangerous," risking expansion of the zone of the conflict and "complete destabilization of the region", Kremlin spokesman Dmitry Peskov told journalists.

Lebanese families displaced from villages farther south slept in shelters hastily set up in schools in Beirut and the coastal city of Sidon. Some who did not find shelter elsewhere slept in cars and parks and on the seaside corniche.

Monday’s heavy bombardment sent thousands fleeing from south Lebanon. Hotels in Beirut were quickly booked to capacity and apartments in the mountains surrounding the capital were snapped up by families seeking safe accommodations.

Iraqi Prime Minister Mohammed Shia al-Sudani has called for an urgent meeting of Arab leaders on the sidelines of the U.N. General Assembly after Israel intensified its strikes on Lebanon.

Iraq "calls on and works to convene an urgent meeting of the leaders of Arab delegations... to review the repercussions of the Zionist (Israeli) aggression on our peaceful people in Lebanon and to work jointly to stop its criminal behavior", Sudani said in a statement on Monday.

France has requested an emergency U.N. Security Council meeting to discuss Lebanon after Israel launched a major cross-border attack following nearly a year of clashes with Hezbollah.

"I have requested that an emergency meeting of the Security Council be held on Lebanon this week," French Foreign Minister Jean-Noel Barrot told the United Nations General Assembly on Monday, calling on all sides to "avoid a regional conflagration that would be devastating for everyone," especially civilians.