Vice President JD Vance arrived in Switzerland with his pregnant wife on Sunday along with top Iranian officials to begin negotiations with Tehran, but renewed conflict between Israel and Hezbollah in Lebanon could derail the talks. 

The US and Iran signed a fragile interim deal to end the war on Wednesday. It gives the two countries 60 days to negotiate a deal to keep the Strait of Hormuz open and establish a path forward on Iran's nuclear program.

The deal also calls for an end to fighting in Lebanon - a key demand from Iran listed in the first paragraph of the agreement. 

But only days after signing the agreement, it is being stress-tested after fighting escalated in Lebanon between Israel and the Iranian-backed militant group Hezbollah.

In response, Iran's military soon announced that the Strait of Hormuz was once again closed. The country said that resolving the situation in Lebanon would become a part of the negotiations in Switzerland. 

A renewed ceasefire in Lebanon, brokered on Saturday, appeared to be holding up as Vance began discussions with Pakistani Prime Minister Shehbaz Sharif and Staff Field Marshall Asim Munir. 

Munir has served as a key intermediary between Washington and Tehran throughout the conflict.

'What´s up, man! Good to see you,' Vance said as he warmly greeted Munir, who serves as Pakistan's army chief.

Sharif met separately with Iranian Parliament Speaker Mohammad Bagher Qalibaf, who is leading Tehran's delegation, and Foreign Minister Abbas Araghchi. 

Mediators from Qatar were also on hand at the mountainside resort near Lake Lucerne.

Rafael Grossi, chief of the U.N. nuclear watchdog, the International Atomic Energy Agency, met with Swiss Foreign Minister Ignazio Cassis on the sidelines of the gathering. 

The agency had monitored the 2015 nuclear deal negotiated between the U.S. and Iran under the Democratic Obama administration. Trump, a Republican, withdrew the US from the agreement in 2018.

Iran´s main focus during the negotiations will be the ongoing war between Israel and Lebanon, Iranian Foreign Ministry spokesman Esmail Baghaei told Iran´s state news agency.

Iran is insisting that the deal´s implementation start with the part of the deal that calls for a cessation of all wars, including between Israel and Hezbollah. Baghaei said the US 'has been unable or unwilling' to hold Israel to the ceasefire.

Iran is cautiously approaching the negotiations given its previous experience with the US negotiations on the nuclear issue, which twice in the past year have been interrupted by massive strikes against the country.

'The implementation of any document is more important than its signing,' Baghaei said Sunday.

Iran´s president has also emphasized that Iran will maintain its right to a nuclear program.

'What is certain is that we will never back down from the right to enrich uranium, and the other side is also forced to accept it,' Iranian President Masoud Pezeshkian said on Sunday, according to Iran´s state media.

Vance had originally been slated to be on the ground at the Bürgenstock resort near Lucerne on Friday, but his departure from the United States was delayed after fighting escalated in Lebanon and Iranian officials canceled plans to attend the talks.

US Central Command disputed Iran´s claim that it had once again shuttered the strait and said US forces continued to monitor the situation to ensure traffic continues to flow through the waterway. 

Vance has said that millions of barrels of oil have moved through the strait in recent days.

This is a developing news story. More to follow.