Israeli Prime Minister Benjamin Netanyahu stepped up pressure on the United States to pull out of a 2015 nuclear deal with Iran, holding a prime-time address on Israeli TV to present what he called evidence of a secret Iranian nuclear weapons program.

Most of the purported evidence Netanyahu unveiled dated to the period before the 2015 accord was signed, although he said Iran had also kept important files on nuclear technology since then, and continued adding to its "nuclear weapons knowledge."

Intelligence experts and diplomats said he did not seem to have presented a "smoking gun" showing that Iran had violated the terms of the agreement, although he may have helped make a case on behalf of skeptics in the U.S. administration who want to scrap it.

Tehran dismissed Netanyahu as "the boy who cried wolf," and called his presentation propaganda.

President Donald Trump has threatened to pull the United States out of the international deal unless it is renegotiated by May 12. He repeated his criticism of the deal after Netanyahu spoke, and suggested he approved of the Israeli leader's remarks.

Live Broadcast: A Special Address by Israeli Prime Minister Benjamin Netanyahu <a href="https://t.co/gll2fPI2eE">https://t.co/gll2fPI2eE</a> —@netanyahu

"Iran's leaders repeatedly deny ever pursuing nuclear weapons," Netanyahu said at Israel's Defence Ministry, standing in front of stacks of files representing what he described as a vault full of Iranian nuclear documents obtained weeks before.

"Tonight I'm here to tell you one thing: Iran lied.

"Iran lied about never having a nuclear weapons program," he said. "100,000 secret files prove it did. Second, even after the deal, Iran continued to preserve and expand its nuclear weapons knowledge for future use," Netanyahu said.

Although the presentation was live on Israeli television, Netanyahu made clear that his audience was abroad: he delivered most of his speech in English, before switching to Hebrew.

'No new and credible evidence'

Tehran has denied ever seeking nuclear weapons and accuses its arch-foe Israel of stirring up world suspicions against it.

However, much of what Netanyahu presented is unlikely to surprise world powers, which have long concluded that Iran was pursuing atomic weapons before the agreement was signed in 2015: that is why they imposed sanctions in the first place.

Washington's European allies say Tehran has generally abided by the terms of the deal since then, and have urged Trump not to scrap it. Some independent analysts and diplomats said Netanyahu appeared to be presenting old evidence.

Netanyahu is telling us something we already knew -- that Iran had a nuclear weapons program. If the JCPOA collapses, Iran will be free to resume that program without additional <a href="https://twitter.com/iaeaorg?ref_src=twsrc%5Etfw">@iaeaorg</a> safeguards imposed by the deal. These documents are a commercial for keeping the JCPOA. —@ArmsControlWonk

"Netanyahu is telling us something we already knew — that Iran had a nuclear weapons program," Jeffrey Lewis, director of the East Asia Nonproliferation Program at the Middlebury Institute of International Studies, said on Twitter.

A senior European diplomat told Reuters: "We knew all of this and what especially stands out is that Netanyahu doesn't speak of any recorded violations of the [2015 Iran deal]."

Speaking after Netanyahu's presentation, Trump told a White House news conference the nuclear deal was "a horrible agreement for the United States." He said it would let Tehran develop nuclear arms after seven years and had "proven right what Israel has done today" with Netanyahu's disclosures.

U.S. President Donald Trump speaks during a news conference with Nigerian President Muhammadu Buhari in the Rose Garden of the White House in Washington on Monday. (Carolyn Kaster/Associated Press)

However, Washington itself has concluded that Iran has not violated the terms of the deal, and two U.S. intelligence officials who have monitored Iran's nuclear weapons program for years said on Monday nothing in Netanyahu's remarks appeared to contradict that view.

"We have seen no new and credible evidence that Iran is violating the agreement, whether in the prime minister's remarks today or from other sources," said one of the officials, both of whom spoke on the condition of anonymity.

'Boy who can't stop crying wolf'

Moments before Netanyahu spoke Iranian Foreign Minister Mohammad Javad Zarif tweeted: "The boy who can't stop crying wolf is at it again."

The head of Iran's Atomic Energy organization said on Monday that Iran has the technical capability to enrich uranium to a higher level than it could before the multinational deal was reached.

"Technically, we are fully prepared to enrich uranium higher than we used to produce before the deal was reached... I hope Trump comes to his senses and stays in the deal," Ali Akbar Salehi was quoted by Iranian state television as saying.

Israel is widely believed to be the only nuclear-armed state in the Middle East, though it neither confirms nor denies possessing atomic weapons.