"One depressing impression I had, and I think it's shared by so many people in Lebanon -- there was a sense of despair and despondency and anxiety," Fletcher told a press conference via videolink from Syria, after a brief trip to Lebanon.

"There's a sense that this is likely to be a long-running conflict, and that even if we reach some sort of moment where the actors declare victory on the US-Israel-Iran conflict, that that may not bring to an end to the Israel-Hezbollah conflict anytime soon."

He said one of his major worries was "a real danger of a fresh occupation, of a fresh occupied territory in southern Lebanon."

Fletcher also expressed fears about the possible fracturing of "national cohesion" in Lebanon, noting "the importance of all Lebanese parties pulling together in this moment of real fragility and concern, as so many people are on the move."

Lebanon was sucked into the Middle East war after Tehran-backed militant group Hezbollah launched rockets at Israel to avenge the US-Israeli attack that killed Iran's supreme leader Ayatollah Ali Khamenei.

Israel has responded with massive strikes across Lebanon and a ground offensive.

Lebanese authorities say the hostilities have so far killed more than 1,300 people and displaced more than a million others.