Washington | The White House said Thursday night that Vice President JD Vance was delaying a trip to Switzerland, where he'd been set to lead a new round of negotiations with Iran over its nuclear programme - raising questions about what's next for the tentative agreement to end the war.

The team led by Vance had been ready to leave but was postponing, the White House said, citing difficult logistics for negotiations.

The announcement followed a report from Al-Mayadeen, a pan-Arab satellite channel that is politically allied with the Iranian-backed Lebanese militant group Hezbollah, that Iran was delaying sending its delegation to Switzerland over Israel's ongoing military campaign in Lebanon.

Vance, who was initially personally sceptical of the US going to war with Iran, has increasingly become the administration's face of the conflict and has been outspoken in defending the deal.

Earlier Thursday, he took the relatively unusual step of appearing at the White House to defend the initial deal to extend the ceasefire 60 days and allow for more negotiating - arguing that while it offers concessions, Iran first has to comply with US demands.

"As they dial up their good behaviour, we can dial up the economic relief," Vance said. "If they dial down their good behaviour, we can turn it off." But the vice president also had said during those remarks that he was not sure of the timing of his planned trip to Switzerland and that talks might not begin this week. The formal postponement now makes that even less clear.

Vance staying put in Washington came after the US said it had lifted its blockade, allowing oil tankers to begin freely moving through the Strait of Hormuz after months of being unable to use the critical channel.

Still, the tentative agreement has drawn sharp criticism from some in the US - including a few congressional Republicans - who worry Washington ceded too much to Iran with relief from sanctions and a potential USD 300 billion fund to help with rebuilding.

Earlier, a top Trump administration envoy told US lawmakers in a private briefing that Iran will invite the UN's nuclear watchdog agency to inspect its nuclear sites.

And Iranian Supreme Leader Ayatollah Mojtaba Khamenei had seemed to endorse direct negotiations for his officials.

"It is obvious that the face-to-face negotiations that will be held in the future will not mean accepting the enemy's opinion," he said in a statement read by state media.

It was Khamenei's first reaction to the agreement, and it was interpreted as a shift in Iran's approach. Hard-liners, especially Khamenei's father, the previous supreme leader, have long opposed direct talks, especially after the US pulled out of the 2015 nuclear deal between Iran and world powers.

The supreme leader has not been seen in public since he was wounded in a strike at the start of the war.

The agreement states that Iran's stockpile of highly enriched uranium must at minimum be diluted under international supervision. It also says that Iran shall not procure or develop nuclear weapons - a commitment it has made previously.

Trump envoy Steve Witkoff told members of Congress that Iran will invite the International Atomic Energy Agency to inspect its nuclear sites and begin work on identifying and uncovering the locations of Tehran's enriched material, which is believed to be buried under rubble.

Witkoff's private briefing was described by two people familiar with the conversation who spoke to The Associated Press on condition of anonymity to share the closed-door details.

The agreement requires Iran to "commit to renounce their nuclear ambitions in writing," said White House spokeswoman Olivia Wales. The IAEA did not respond to a request for comment.

Witkoff told congressional leadership and members of national security-related committees that the agreement the US struck with Iran did not include any side deals, but a side letter was drafted between Tehran and the IAEA extending the invitation.

Witkoff said the letter to IAEA Director-General Rafael Mariano Grossi would enable him to bring US nuclear inspectors to Tehran.

Before Vance delayed his trip, Pakistani Prime Minister Shehbaz Sharif postponed a planned visit to Switzerland, where Islamabad officials had originally planned to host a ceremonial signing ceremony for the agreement.

That visit was postponed because the agreement had already been signed by both Iran and the US, said two senior officials who spoke on condition of anonymity because of the sensitivity of the matter.

President Donald Trump signed the initial pact with Iran on Wednesday while dining with French President Emmanuel Macron at the Palace of Versailles. The deal is slated to take immediate effect and extends a ceasefire while giving each side 60 days to hammer out broader agreements on larger issues.

Vance, in his comments at the White House, shrugged off criticism about the confusing rollout of the initial deal, saying, "I don't think our public messaging has been chaotic." He also offered a blunt warning to Israel, which has pushed the US to take a harder stance against Iran and launched attacks on the Iranian-backed Hezbollah militia in Lebanon throughout the war, including just before the deal extending the ceasefire was reached. Those attacks complicated the peace efforts with Iran.

Trump "is the only head of state in the entire world who is sympathetic to the nation of Israel at this moment in time," Vance said. "And he happens to be the head of state of the world's superpower." Trump said he signed the agreement to avoid "economic catastrophe" in the US, after the war caused oil prices to skyrocket, made financial markets skittish and fuelled inflation.

The deal caused gas prices to fall and stock markets to rise - though rallies could be threatened again depending on how the next round of US-Iran talks go.

The vice president said more than 12.5 million barrels of oil went through the Strait of Hormuz on Wednesday night and said that the US easing its blockade of Iran means "honouring our end of the early part of the agreement on the military side." US Central Command said American warships "will remain in the general area to make sure that all aspects of the agreement are adhered to, obeyed and in full force and effect." Iranian state media said shipping had "normalised" at Iran's southern ports but added that the strait remains supervised and under the control of the Iranian military, and transiting through the vital waterway still requires coordination.

Major shipowners began moving vessels through the strait after the agreement was signed, according to maritime data company Lloyd's List Intelligence, though Lloyd's did not give data on how many ships have passed through the strait as of Thursday.

In a media briefing, Richard Meade, editor-in-chief of Lloyd's List, said for the first time in 110 days, ships owned by major companies are transiting the strait after effectively being marooned there since February.

It could take weeks or months to fully reopen the strait, and the two alternative routes do not have as much capacity as the strait's central passage. (AP)