He spoke shortly after Washington and Tehran agreed a two-week truce to the war, announced before Trump's deadline to agree a deal or face obliteration.

Tehran has warned the war is not over until formal terms are negotiated.

"If the Iranians are willing in good faith to work with us, I think we can make an agreement," Vance said during a visit to Budapest.

He added if the Iranians don't come to the negotiating table, "they're going to find out that the President of the United States is not one to mess around. He's impatient. He's impatient to make progress."

Vance said Trump had shown "that we still have clear military, diplomatic, and maybe most importantly we have extraordinary economic leverage".

"If they're going to lie, if they're going to cheat, if they're trying to prevent even the fragile truce that we've set up from taking place, then they're not going to be happy," he added.

Vance started his Budapest visit on Tuesday to support Hungarian nationalist Prime Minister Viktor Orban, who is facing an unprecedented challenge to his 16-year rule in Sunday's parliamentary elections.