According to Iran's Tasnim News Agency, citing sources, the Iranian delegation departed from the Swiss venue after Trump's warning of fresh military action against Tehran. Iran's semi-official Fars News Agency also reported that negotiations had been halted following the US president's remarks, leaving the future of the talks uncertain.

Iranian delegation rejects pre-talk gestures

Reports said organisers and members of the US delegation had planned a handshake and joint photo session between the two sides before the talks began. However, the Iranian delegation, led by chief negotiator Mohammad Bagher Ghalibaf, declined to participate.

The negotiations, aimed at easing tensions between Washington and Tehran, were already facing significant challenges before the latest escalation.

Trump warns Iran over Hezbollah

Earlier on Sunday, Trump warned Iran to restrain its allies in Lebanon, particularly Hezbollah, or face stronger military action.

In a post on Truth Social, Trump wrote: "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!!! President DONALD J. TRUMP."

The remarks referenced recent US military action and signalled the possibility of further strikes if Washington believed Tehran-backed groups continued destabilising activities in the region.

Ghalibaf hits back at US president

Responding to Trump's comments, Ghalibaf dismissed the threats and warned that Iran's armed forces were prepared to retaliate.

"Don't they think to themselves that if their threats had any effect, they wouldn't have reached the point of despair today? We don't count on the threats of the Americans," Ghalibaf wrote on X.

He further cautioned Washington against escalating its rhetoric, saying: "They better be careful with their statements, our armed forces are ready to respond in another way. Whatever they say, we are the ones who will act."

Vance speaks of a 'new leaf'

Before the talks were thrown into uncertainty, US Vice President JD Vance had struck an optimistic tone while speaking briefly to reporters ahead of negotiations with senior Iranian officials.

"The question before us now is how much more can we accomplish together? Can we turn over a new leaf?" Vance said as discussions, dubbed the "Lake Lucerne Summit", got underway.