President Trump has threatened further attacks on Iran while Vice President Vance attended talks with Iranian officials in Switzerland on Sunday.
"Iran must immediately stop their highly paid PROXIES in Lebanon from causing trouble. If they don't, we'll hit Iran very hard again, just like we did last week, only harder!!!" Trump wrote in a social media post on Sunday.
A day earlier, Iran's military announced it had closed the Strait of Hormuz because of continued Israeli strikes in Lebanon against the Iranian-backed militant group Hezbollah. Iran said the U.S. failure to rein in Israel violated the terms of last week's tentative agreement, which specifies that all fighting in Lebanon must end.
Also on Sunday, Vance, who arrived in Switzerland in the early morning, met with representatives from Pakistan who have been brokering the talks – including with the country's Prime Minister Shehbaz Sharif and Staff Field Marshal Asim Munir.
An Iranian team, mediators from Qatar and Rafael Grossi, chief of the International Atomic Energy Agency, are also attending the meetings.
The talks are focused on a Memorandum of Understanding that was signed by both the U.S. and Iran last week, but which is already coming under intense strain. While Iran on Saturday said it had closed the Strait of Hormuz, U.S. Central Command has said shipping through the strait was proceeding normally.
Also under discussion is Iran's nuclear program, another controversial topic. On Sunday morning, Iranian president Masoud Pezeshkian said: "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". Iran has insisted its nuclear program is peaceful.
Despite announcing a ceasefire on Friday, Israeli forces and Hezbollah exchanged heavy fire throughout Saturday, further putting pressure on the negotiations.
Despite the tensions, Vance claimed the talks were going well, telling reporters on Sunday: "We've already made great progress over just the last few hours, and I expect that we'll make additional progress in the hours to come."
Vance said there has been "great progress" in the last couple of days and "these things are always a little bit messy" when asked if he had a message for Israeli Prime Minister Benjamin Netanyahu.
"There, of course, are going to be sometimes disagreements about precisely how to get there, but I actually feel great about where we are in Lebanon. There's still some additional wood to chop, but we're going to keep on working," Vance said.
The Vice President also said the U.S. has "done more to stop the conflict in Lebanon than any government anywhere in the world."
At least 16 people, including civilians, were killed by Israeli strikes on Saturday, according to the Lebanese National News Agency. Israel said the strikes were a response to Hezbollah firing projectiles at its forces overnight on Saturday. Hezbollah said it fired in response to Israel moving toward Lebanese territory.
On Sunday, however, the interim head of the United Nations Interim Force in Lebanon told NPR that for the first time since the war between Israel and Hezbollah started on March 2, the peacekeeping force has recorded no attacks from either side.
Neither Israel or Lebanon have signed the Memorandum of Understanding, but the agreement calls for respect of Lebanese sovereignty, a provision Iran says the U.S. must enforce. It also calls for a halt to military operations in Lebanon.
NPR's Jane Arraf contributed to this report.
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