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.
France's Foreign Minister Jean-Yves Le Drian said it was "not dead", 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."
How do key powers see Mr Trump's decision?
In his comments to French radio, Mr 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.
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, President 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".
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.
Mr Trump also said he would reimpose economic sanctions that were waived when the deal was signed in 2015.
The US Treasury said the sanctions would target industries mentioned in the deal, including Iran's oil sector, aircraft manufacturers exporting to Iran and Iranian government attempts to buy US dollar banknotes.
Major European and US companies are likely to be hit. Some exemptions are due to be negotiated but it is not yet clear what.
US National Security Adviser John Bolton is reported as saying that European companies doing business in Iran will have to stop doing so within six months or face US sanctions.
What was agreed under the deal?
The JCPOA saw Iran agree to limit the size of its stockpile of enriched uranium - which is used to make reactor fuel, but also nuclear weapons - for 15 years, and the number of centrifuges installed to enrich uranium for 10 years.
Iran also agreed to modify a heavy water facility so it could not produce plutonium suitable for a bomb.
In return, sanctions imposed by the UN, US and EU that had crippled Iran's economy were lifted.
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.
President Rouhani has warned that Iran could resume "industrial-scale" uranium enrichment if the agreement collapses.
After the deal was implemented, it was estimated it would take Iran at least 12 months to gather enough highly enriched uranium or plutonium to build a nuclear weapon if it chose to do so.
But experts say that it could still take Iran years to acquire a working nuclear arsenal.