Iran's supreme leader, Ayatollah Ali Khamenei, has addressed US President Donald Trump directly, telling him he "made a mistake" in deciding to leave a multi-country nuclear deal.
Mr Khamenei said: "I said from the first day: don't trust America."
He urged his government to get guarantees from European powers before agreeing to continue with the deal.
The 2015 agreement curbed Iran's nuclear activities in return for the lifting of UN, US and EU sanctions.
It was signed by Mr Trump's predecessor, Barack Obama, between Iran and the five permanent members of the UN Security Council - the US, UK, France, China and Russia - plus Germany.
But Mr Trump, long opposed to the deal, said on Tuesday that it was "defective at its core" and the US would withdraw, reimposing economic sanctions.
Other signatories to the accord say they are still committed to it and want it to continue.
French President Emmanuel Macron has also called the US decision "a mistake". The UK's Foreign Secretary Boris Johnson said Britain had "no intention of walking away", and Germany's Chancellor Angela Merkel said her country would "do everything in our power for Iran to adhere to its obligations in the future".
Iran's President Hassan Rouhani, who is regarded as a moderate reformist and under whom the 2015 deal was signed, has indicated he will try to salvage it.
He said his country was preparing to restart uranium enrichment in case the deal collapsed, but also said: "If we achieve the deal's goals in co-operation with other members of the deal, it will remain in place."
But Mr Khamenei, the highest power in the land, was sceptical, saying in a televised speech that he did not trust Britain, France or Germany, and would need "guarantees" before continuing the nuclear deal.
Addressing the government, he said: "We hear that you want to continue the nuclear deal with the three European countries. I don't have confidence in these three countries."
"If you want to conclude an agreement, obtain real guarantees, otherwise tomorrow they will do the same as the United States."
"Their words have no value," he said of foreign leaders.
"Today they say one thing and tomorrow another. They have no shame."
Meanwhile, Mr Trump warned Iran of unspecified "severe consequences" if it restarted its nuclear programme.
Addressing reporters at the White House on Wednesday, he said he hoped Iran would be willing to negotiate a new deal, adding: "I would advise Iran not start their nuclear programme. I would advise them very strongly. If they do there will be very severe consequence."
Sanctions 'target people, not politicians'
By Rana Rahimpour, BBC Persian service
The Iranians seem divided on social media. The tag #thankyoutrump has been used by those who are supportive of the US president's decision to leave the nuclear deal with Iran.
Those who are happy are from different groups. They include the hardline anti-American opponents of President Rouhani, plus the people that are against the regime who say the deal hasn't benefited them.
And then there are those in the middle, those who do want change but are not sure that sanctions will accelerate that. They sound disappointed. They accuse the US of being deceptive and dishonest. They say the sanctions will target the people not the politicians.
How do key powers see Mr Trump's decision?
Mr Macron said France - "along with our partners" - was looking to prevent tensions escalating in the region.
"I regret the decision of the American president. I think it's a mistake and that's why we Europeans have decided to remain in the nuclear agreement," he told German broadcaster ARD.
In earlier comments, France's Foreign Minister Jean-Yves Le Drian said: "The deal is not dead. There's an American withdrawal from the deal but the deal is still there".
He said there would be a meeting between France, Britain, Germany and Iran on Monday.
Russia said it was "deeply disappointed" by Mr Trump's decision, while China and Germany expressed regret.
But the move has been welcomed by Iran's major regional rivals, Saudi Arabia and Israel.
Saudi Arabia signalled on Wednesday that it would look to acquire its own nuclear weapons if Iran does.
"We have made it very clear that if Iran acquires a nuclear capability, we'll do everything we can to do the same," foreign minister Adel al-Jubeir told CNN.
Israeli Prime Minister Benjamin Netanyahu, a prominent critic of the accord, said he "fully supports" Mr Trump's withdrawal from a "disastrous" deal.
Why did the US withdraw?
In his address on Tuesday, Mr Trump called the nuclear accord - or Joint Comprehensive Plan of Action (JCPOA) as it is formally known - a "horrible, one-sided deal that should have never, ever been made".
Mr Trump alleged that the deal did not restrict Iran's "destabilising activities" in the region enough, and could not detect or prevent any breaking of the terms of the deal.
Iran insists its nuclear programme is entirely peaceful, and its compliance with the deal has been verified by the International Atomic Energy Agency (IAEA), who on Wednesday again said Iran was honouring its commitments.
- limiting the size of its stockpile of enriched uranium, which is used to make reactor fuel but also nuclear weapons
- limiting the number of centrifuges installed to enrich uranium
- modifying a heavy water facility so it could not produce plutonium suitable for a bomb
But Mr Trump said: "It didn't bring calm, it didn't bring peace, and it never will."
He said he would work to find a "real, comprehensive, and lasting" deal that tackled not only the Iranian nuclear programme but its ballistic missile tests and activities across the Middle East.
He also said he would reimpose economic sanctions that were waived when the deal was signed in 2015.
How will sanctions affect Iran?
The US Treasury said sanctions, coming in after a wind-down period of 90-180 days, would target industries mentioned in the deal, including Iran's oil sector and government attempts to buy US dollar banknotes.
Some exemptions are due to be negotiated but it is not yet clear what they will be.
Iran's currency, the rial, fell in the run-up to Mr Trump's announcement and afterwards, heralding economic pain for the country.
While Iran was under sanctions between 2012 and 2016, its oil production fell. On Tuesday, oil prices rose worldwide to a three-and-a-half-year high, in anticipation of reduced supply. Saudi Arabia said it would step in to prevent shortages.