"We're on the right pathway," Vance told the Munich Leaders Meeting in Washington.

"We don't care if people want nuclear power. We're fine with that, but you can't have the kind of enrichment program that allows you to get to a nuclear weapon, and that's where we draw the line," he said.

Asked later Wednesday if Iran can maintain an enrichment program as long as it does not lead to a weapon, US President Donald Trump said that remained an open question.

"We haven't made that decision yet. We will, but we haven't made that decision yet," Trump said at the White House.

Iran and the United States have engaged in nuclear talks since April 12, their highest-level contact since Washington withdrew from a landmark deal with Tehran in 2018, during Trump's first term as US president.

A fourth round of talks initially scheduled for May 3 was postponed, mediator Oman said, citing "logistical reasons."

The United States and other Western countries have long accused Iran of pursuing nuclear weapons -- a claim Tehran denies, insisting that its atomic program is solely for civilian purposes.