GENEVA : A newly signed deal to end the Middle East war was already under strain on Friday, after talks in Switzerland were postponed and fighting flared between Israel and Hezbollah in Lebanon.

The developments came after Iran’s supreme leader announced he had only allowed the preliminary deal to the stop war to go ahead despite reservations, and as his top negotiator warned Washington the Islamic republic stood ready to retaliate in the event of any breach.

Mediators in the conflict—including Pakistan, Saudi Arabia and Turkey—were due to gather for talks in the Egyptian city of Alamein on Sunday to discuss the deal, Cairo and Islamabad said.

Preparations had been made to host Iranian and US delegations at the Swiss resort of Burgenstock, overlooking Lake Lucerne, to begin negotiations on implementing the deal signed this week by President Donald Trump and his Iranian counterpart Masoud Pezeshkian.

Iran’s top negotiator Mohammad Bagher Ghalibaf warned that Tehran would deliver a “decisive response” in the event of “breach of contract” or “excessive demands”.

“They were once slapped during the war; if they wish to head on that path again, they will get an even harder slap,” he wrote on X.

The signing of the accord was intended to end the US-Israeli campaign against Iran—which saw five weeks of all-out war until a ceasefire was struck in April—and reopen the Strait of Hormuz, the key shipping bottleneck whose closure caused global energy prices to rise.

The agreement also kicks off a 60-day period for talks on wider issues, including Tehran’s nuclear programme.

Ghalibaf and US Vice President JD Vance had been expected in Burgenstock along with Pakistani and Qatari mediators on Friday to commence the process.

“The planned talks between the US, Iran, Qatar and Pakistan have been postponed,” the Swiss foreign ministry said in a message to AFP.

“Switzerland remains ready to facilitate these talks. The relevant preparatory work at Burgenstock is continuing,” it added, without providing a new date for the talks.

It followed the announcement late Thursday from the White House that Vance’s trip was cancelled, with a spokesperson saying the “logistics of these negotiations have never been simple or predictable”.

“We look forward to beginning technical talks as soon as possible.”

The deal was also meant to halt the fighting in Lebanon, but Israel’s military announced on Friday new strikes against Hezbollah targets, with 18 people killed, according to the Lebanese health ministry.

Israel also said that four of its soldiers were killed in southern Lebanon, the first since the deal was signed.

The soldiers’ deaths prompted a furious reaction from far-right Israeli National Security Minister Itamar Ben Gvir, who demanded that “all of Lebanon must burn”.

Iran’s supreme leader Ayatollah Mojtaba Khamenei said Thursday that he had approved the accord, despite holding a “different view”.

“But I issued my permission due to the commitment” made by officials including Pezheshkian to “protect the rights of the Iranian nation”.

Khamenei—who has yet to be seen in public since succeeding his father, who was killed in US-Israeli strikes—said Trump had “used all kinds of levers” to secure the deal “out of desperation”.