US Secretary of State Mike Pompeo says a landmark nuclear deal with Iran was "built on lies", after Israel claimed to have proof of a secret Iranian nuclear weapons programme.
He said it showed that the accord with Iran and six world powers in 2015 was based on deception, and not goodwill.
President Trump has long signalled his desire to abandon the deal and is due to make a decision in the coming weeks.
Other Western powers say Iran has kept to the deal and it should be continued.
What is Iran accused of?
Israeli Prime Minister Benjamin Netanyahu on Monday accused Iran of conducting a secret nuclear weapons programme, dubbed Project Amad, and said it had continued to pursue nuclear weapons knowledge after the project was shuttered in 2003.
He presented what he said was evidence of thousands of "secret nuclear files" that showed Iran had lied about its nuclear ambitions before the landmark deal was signed in 2015.
Iran has always denied seeking nuclear weapons, and agreed three years ago to curb its nuclear energy programme in return for the lifting of sanctions.
The Israeli prime minister did not provide evidence that Iran had violated the accord since it went into effect in early 2016. But he insisted that Project Amad had continued at the Iranian defence ministry - citing the head of the programme as saying, "Special activities will be carried out under the title of scientific know-how developments."
Iran's Foreign Minister Javad Zarif said the move by Mr Netanyahu was a "childish" stunt to influence Mr Trump's decision on whether the US should stick to a nuclear deal with Iran.
A spokesman for the UK government, a signatory of the deal, said it would continue to back the deal, adding: "We have never been naive about Iran and its nuclear intentions."
Rob Malley, who was on the Iran negotiating team under the Barack Obama administration, played down the allegations, saying they were "nothing new".
For those who have followed the Iranian nuclear file, there is nothing new in Bibi's presentation. All it does is vindicate need for the nuclear deal— Rob Malley (@Rob_Malley) April 30, 2018
But the Israeli prime minister has an audience of one: Trump
And he's unfortunately unlikely to reach the same conclusion.
What does the US say?
"The documents obtained by Israel from inside Iran show beyond any doubt that the Iranian regime was not telling the truth," Mr Pompeo said in a statement.
"We assess that the documents we have reviewed are authentic," he said, adding: "Iran hid a vast atomic archive from the world and from the International Atomic Energy Agency (IAEA) - until today."
Mr Pompeo also warned that the US was now "assessing what the discovery of Iran's secret nuclear files means for the future".
Mr Trump, who has been vocal about his opposition to the Obama-era deal, said he had viewed part of Mr Netanyahu's presentation and said the situation was "not acceptable".
He said he would make a decision on whether to retain the deal on or before 12 May.
What 'proof' did Netanyahu produce?
Speaking in English from Israel's defence ministry in Tel Aviv, Mr Netanyahu showed off what he said were "exact copies" of documents obtained by Israeli intelligence from a secret storage facility in Tehran.
There were, he said, 55,000 pages of evidence and a further 55,000 files on 183 CDs relating to Project Amad.
The project, he said, had had the explicit goal of producing five warheads, each with the yield of 10 kilotonnes of TNT.
"These files conclusively prove that Iran was brazenly lying when it said it never had a nuclear weapons programme."
The files had been shared with the US, Mr Netanyahu said, and would be submitted to the IAEA.
A 2007 US National Intelligence Estimate assessed "with high confidence" that Iran did have a nuclear weapons programme up until 2003 but that Iran had stopped it after its discovery.
On Monday, the Israeli prime minister argued the existence of the alleged files proved Iran had been "secretly storing Project Amad material to use at a time of its choice to develop nuclear weapons".
How is the 2015 deal meant to work?
The agreement signed between Iran, the US, China, Russia, Germany, France and Britain lifted crippling economic sanctions in return for curbs on Tehran's nuclear programme.
There had been fears that Iran would use the programme to create a nuclear weapon.
Under the deal, officially known as the Joint Comprehensive Plan of Action (JCPOA), Iran is committed to slashing the number of its centrifuges, which are machines used to enrich uranium.
It is also meant to cut its stockpile of enriched uranium drastically and not enrich remaining uranium to the level needed to produce nuclear weapons.
How dangerous is the enmity between Israel and Iran?
Tension between the long-standing enemies has grown steadily since Iran built up its military presence in Syria, Israel's north-eastern neighbour.
Iran has also been accused of supplying weaponry to Lebanese Shia Muslim militant group Hezbollah, an enemy of Israel, and also smuggling arms to Palestinian militants.
Mr Netanyahu has long vowed to stop Iran from strengthening its military presence in Syria.
On Sunday night, a wave of unclaimed air strikes on targets in Syria reportedly killed a number of Iranians.
Sites allegedly linked to a covert Syrian chemical weapons programme were bombed by Western nations earlier this month.
Israel has also carried out, or is believed to have carried out, dozens of air strikes on facilities in Syria used by Iranian forces.
The number of centrifuges installed at Iran's Natanz and Fordo sites was cut drastically soon after the deal while tonnes of low-enriched uranium were shipped to Russia.
Furthermore, monitors from the IAEA have been able to carry out snap inspections at Iranian nuclear sites.