Israel-Gaza-Lebanon live updates: 20 dead in Israeli strike on school, ministry says

American citizens in Lebanon "are strongly encouraged to depart now," the U.S. Embassy in Beirut said in a new alert Monday.

The embassy has been urging Americans to depart Lebanon via commercial flights in recent weeks. Monday's warning was the starkest yet.

The embassy noted it had helped add thousands of extra seats to commercial flights to help Americans leave amid the escalating conflict between Israel and Hezbollah.

"Much of this capacity has gone unused," Monday's alert said. "Please understand that these additional flights will not continue indefinitely."

"U.S. citizens who choose not to depart at this time should prepare contingency plans should the situation deteriorate further," the embassy said.

"These alternative plans should not rely on the U.S. government for assisted departure or evacuation," the notice read.

The embassy has been warning citizens not to travel to Lebanon since July.

Eighteen people were killed and four wounded in an airstrike in the town of Aitou in northern Lebanon on Monday, the Lebanese Red Cross wrote on X.Seven Red Cross teams were dispatched to the area in the Zgharta district, the organization said. "Our teams are working to provide first aid and evacuate the wounded," it added.

-ABC News' Ghazi Balkiz and Guy Davies

Hezbollah issued 38 statements claiming cross-border attacks into Israel on Sunday -- the highest tally since renewed fighting began on Oct. 8, 2023, per ABC News' count.

The attacks included the drone strike on an Israel Defense Forces training base in northern Israel, which killed four soldiers and injured 55.

Hezbollah has expanded its attacks into Israel despite the IDF's monthslong campaign of targeted killings of top commanders and airstrikes on Hezbollah military facilities and weapons caches.

-ABC News' Ghazi Balkiz and Guy Davies

The Israel Defense Forces said Monday it killed a Hezbollah commander responsible for anti-tank missile forces.

The IDF said in a statement posted to social media that Muhammad Kamal Naim was killed in an airstrike in the Nabatieh region of southern Lebanon.

Naim, it said, was responsible for the elite Radwan Force's anti-tank weapons.

Naim "was responsible for planning and carrying out many terrorist plots, including firing anti-tank missiles at the Israeli rear," the IDF wrote.