Vice President JD Vance landed in Switzerland early Sunday for talks with Iranian officials, joining Trump’s special envoy Steve Witkoff and son-in-law Jared Kushner in negotiations.

They are set to meet a senior Iranian delegation to discuss the technical details of a memorandum of understanding signed last week by President Donald Trump in Paris, which has already been tested by fighting between Israel and the Iran-backed militant group Hezbollah in Lebanon.

The Iranians are set to emphasize their “key and primary demand” of ending the war on all fronts, including Lebanon, and “the failure to implement it” will be discussed with mediators, Iranian state TV reported.

Iran said Saturday that it has once again closed the Strait of Hormuz, a vital waterway through which 20% of the world’s oil passes, alleging ceasefire violations by Israel. The U.S. military has denied Iran’s claim to control the strait.

The Islamic Revolutionary Guard Corps Navy warned ships not to approach the waterway, which Iran had committed to reopening under the interim peace deal.

Trump said on Truth Social Saturday, amid uncertainty over whether the waterway is actually open, that there would be “NO TOLLS” on the strait during or after the current 60-day ceasefire, “unless they are imposed by and for the United States of America”.

Trump added that the U.S. could charge tolls for ships passing through the Strait of Hormuz, referring to the US. as the “Guardian Angel” of the Middle East.

But Iran, through various officials and channels, has said that the failure to implement the first clause of the MOU means there is no agreement in effect.

A violation of Article 1, which specified that fighting must end on all fronts, including Lebanon, “calls the entire agreement into question,” Iran’s foreign ministry spokesman, Esmail Baghaei said Saturday, warning that unless the other side immediately adopts the necessary measures, the memorandum as a whole will face serious difficulties.

At least 16 people were killed by Israeli strikes in Lebanon on Saturday, according to the country’s health ministry, after strikes on Friday killed 83.

A statement from the Israel Defense Forces said Hezbollah had breached the ceasefire and “launched more than 50 projectiles toward IDF soldiers operating in southern Lebanon” overnight, and that Israel had attacked what it described as Hezbollah targets in response. “The IDF remains committed to the ceasefire agreement,” the statement said.

Hezbollah said it had adhered to the ceasefire, accusing Israel of making false claims to justify its attacks in an effort to “sabotage the agreement” between Iran and the U.S.

U.S. spy agencies believe that Israel will likely continue to launch attacks against Hezbollah in Lebanon, according to a source with knowledge of the intelligence assessments.

Speaking to reporters before he boarded the plane to Switzerland, Vance said that the situation in Lebanon had “calmed down” despite news reports, and added, “I think we’re going to hopefully make progress on the nuclear issue, hopefully make progress on the Lebanon ceasefire issue. Those are the two big things that I think we’re going to be focused on.”

Baghaei told Iranian media that the trip to Switzerland was a “mission to demand the fulfillment of the other side’s obligations,” adding that “negotiations for a final agreement” will begin when such obligations are implemented.

Iran’s Parliament Speaker Mohammad Bagher Ghalibaf and Foreign Minister Abbas Araghchi are leading the Iranian delegation.

The talks will be joined by mediators from Qatar and Pakistan. Pakistan Prime Minister Muhammad Shehbaz Sharif arrived in the Swiss city of Zurich on Sunday, along with his delegation.

The current agreement establishes toll-free travel through the strait for 60 days.

Negotiations in Switzerland are also meant to resolve some of the thorniest issues in the deal that are yet to be agreed upon, including Iran’s nuclear program.

According to the memorandum of understanding, Iran has reaffirmed a promise not to develop nuclear weapons — which it did under Obama’s 2015 nuclear deal.