Israeli air strikes on Lebanon killed at least 558 people on Monday, including 50 children, the health ministry said, marking the deadliest day of cross-border violence since the Gaza war began.

Arab states strongly condemned Israel for the escalating hostilities with Hezbollah, which have intensified to levels unseen in nearly a year.

The United States is presenting “concrete” ideas to ease the crisis in Lebanon, a U.S. official said, while voicing opposition to any Israeli ground invasion to target Hezbollah.

“We’ve got some concrete ideas we’re going to be discussing with allies and partners this week to try to figure out the way forward on this,” the senior U.S. official said as world leaders gathered in New York for the U.N. General Assembly.

The U.S. is sending additional troops to the Middle East in response to a sharp spike in violence between Israel and Hezbollah forces in Lebanon that has raised the risk of a greater regional war, the Pentagon said Monday.

Pentagon press secretary Maj. Gen. Pat Ryder would provide no details on how many additional forces or what they would be tasked to do. The U.S. currently has about 40,000 troops in the region.

President Joe Biden said on Monday that the United States was trying to calm the situation in Lebanon after Israeli air strikes on Hezbollah strongholds killed at least 274 people, including children.

"I've been briefed on the latest developments in Israel and Lebanon. My team is in constant contact with their counterparts, and we're working to de-escalate in a way that allows people to return home safely," Biden said as he held talks with UAE President Sheikh Mohamed bin Zayed Al Nahyan at the White House.

Israeli Prime Minister Benjamin Netanyahu warned Lebanese people to move away from danger as Israel pounded Hezbollah targets in the country's south and east on Monday.

"Please, get out of harm's way now. Once our operation is finished, you can come back safely to your homes," Netanyahu said in a video statement shortly after the Israeli army announced it had struck 800 Hezbollah targets in Lebanon.

A source close to Hezbollah said an Israeli strike on Beirut's southern suburbs on Monday targeted the group's commander for south Lebanon, where the movement has clashed with Israel for nearly a year.

"The target of the Israeli strike was Ali Karake, the current commander of the southern front and the third-in-command after" two top commanders who were both killed in Israeli strikes in the southern suburbs in recent weeks, said the source requesting anonymity to discuss sensitive matters.

Israel is bombing targets across many parts of Lebanon, striking senior militants in Beirut's southern suburbs and apparently hiding bombs in pagers and walkie-talkies. Hezbollah is firing rockets and drones deep into northern Israel, setting buildings and cars alight.

But no one is calling it a war — not yet.

Israeli Prime Minister Benjamin Netanyahu said on Monday that the military was changing the "security balance" along the northern border after launching the deadliest strikes on south Lebanon since fighting started last October.

"I promised we would change the security balance, the balance of power in the north –- and that is exactly what we are doing," Netanyahu said during a security meeting, adding that Israel's policy was to preempt threats rather than "wait" for them.

French presidential envoy for Lebanon Jean-Yves Le Drian will arrive in Beirut on Monday, media reports said.

The United Nations voiced alarm Monday at the escalating violence between Israel and Hezbollah in Lebanon, warning that actions and rhetoric was catapulting the Mideast conflict "to another level".

"We are extremely concerned, deeply worried about the escalation in Lebanon," Ravina Shamdasani, spokeswoman for the UN rights office, told AFP.