Iran's Revolutionary Guards threatened to open "new fronts" and keep the Strait of Hormuz closed over Israel's offensive in Lebanon, state media reported on Monday.

The United Nations on Monday expressed its alarm and called for all sides to respect the ceasefire as Israel expanded its offensive into Lebanon, while negotiations to end the U.S.-Iran war appeared in peril.

"We are deeply alarmed by the escalation in military activities across southern Lebanon and beyond," Stephane Dujarric, spokesman for U.N. Secretary-General Antonio Guterres, said.

Israeli Prime Minister Benjamin Netanyahu said on Monday he told U.S. President Donald Trump that Israel would strike Beirut if Hezbollah doesn't stop attacking Israel.

Lebanon's U.S. embassy said on Monday that Hezbollah had accepted a U.S. proposal for a "mutual cessation of attacks", after Israel threatened more strikes on south Beirut on the eve of a fourth round of Israel-Lebanon negotiations.

U.S. President Donald Trump said Monday that talks with Iran were moving ahead rapidly, despite Iran's Tasnim news agency reporting Tehran had halted talks in protest at Israel's Lebnanon offensive.

"Talks are continuing, at a rapid pace, with the Islamic Republic of Iran. Thank you for your attention to this matter!" Trump said on his Truth Social account, shortly after another post saying Israel and Hezbollah had agreed to stop attacks.

U.S. President Donald Trump said Monday he had persuaded Israel and Hezbollah to de-escalate, with Prime Minister Benjamin Netanyahu agreeing not to send warplanes to bomb Beirut's southern suburbs and the Lebanese group promising to stop attacks.

Israel said Monday it would once again target Beirut's southern suburbs, a Hezbollah stronghold mostly spared heavy attacks since April, as it stages its deepest incursion into Lebanon in two decades.

Iran's Tasnim news agency said Tehran was suspending all exchanges with mediators in peace talks with the United States on Monday, blaming Israel's ongoing invasion of Lebanon.

"Given the continuing crimes of the Zionist regime (Israel) in Lebanon and considering that Lebanon was one of the preconditions for the ceasefire and that this ceasefire has now been violated on all fronts, including Lebanon, the Iranian negotiating team is suspending dialogues and exchange of texts through mediators," Tasnim reported.

Hezbollah said its fighters were still battling Israeli troops near south Lebanon's Beaufort castle on Monday, a day after Israel said it seized the strategic fortress and troops raised the Israeli flag there.

The European Union on Monday urged Israel to halt its military operation in Lebanon, after Israel seized the strategic Beaufort Castle and said it would resume strikes on southern Beirut.

"We call on Israel to stop its military escalation in Lebanon and to respect Lebanon's sovereignty and territorial integrity," EU spokesman Anouar El Anouni said.