Iranian President Hassan Rouhani said on Sunday that if the United States quits the nuclear deal between Tehran and world powers Washington would regret it "like never before".
"If the United States leaves the nuclear agreement, you will soon see that they will regret it like never before in history", Mr Rouhani said in a televised speech in northwestern Iran.
The Iranian leader said the Islamic Republic had plans in place to respond to any decision by US President Donald Trump to leave the nuclear agreement signed with world powers in July 2015.
Mr Trump has said unless European allies rectify "flaws" in Tehran's nuclear agreement with world powers by May 12 he will refuse to extend U.S. sanctions relief for Iran.
"We have plans to resist any decision by Trump on the nuclear accord", Rouhani said in a speech carried live by state television.
"Orders have been issued to our atomic energy organisation...and to the economic sector to confront America's plots against our country", Mr Rouhani told the rally.
"America is making a mistake if it leaves the nuclear accord", Rouhani said.
Britain, France and Germany remain committed to the nuclear accord but, in an effort to keep Washington in it, want to open talks on Irans ballistic missile programme, its nuclear activities beyond 2025 – when key provisions of the deal expire – and its role in Middle East crises such as Syria and Yemen.
"We will not negotiate with anyone about our weapons and defenses, and we will make and store as many weapons, facilities and missiles as we need", Rouhani said, reiterating a rejection by Iranian leaders of talks on Iran's missile programme which Tehran says is defensive.
On Sunday, British Foreign Secretary Boris Johnson flew into Washington to hold talks with the Trump administration aimed at preserving the agreement.
French President Emmanuel Macron and German Chancellor Angela Merkel have both called on the US leader to stick to the deal.
The nuclear agreement itself isn't cited as a discussion point in a statement on the trip from Britain's Foreign Office. But Iran is listed as one of the international issues Johnson is to discuss along with North Korea and Syria.
The Foreign Office statement says Johnson is expected to meet with Vice President Mike Pence, national security adviser John Bolton and other senior administration officials as well as congressional foreign policy leaders.
The diplomatic back-and-forths come after Israel last week presented what it said was new evidence of a "secret" nuclear weapons programme that Iran was pursuing. Israeli Prime Minister Benjamin Netanyahu is a strong opponent of the nuclear deal, arguing that he allows Iran to boost its presence on Israel's borders in Lebanon, Syria and Gaza.
European countries, which have been pressing Trump to stick with the deal, said Mr Netanyahu's presentation only reinforced the importance of the agreement, which provides for inspections.
On Sunday, he accused Iran of supplying advanced weapons to Syria that pose a danger to Israel, saying it's better to confront Tehran sooner rather than later.
Israel has repeatedly warned it will not tolerate a lasting Iranian military presence in neighboring Syria, and is believed to have been behind recent airstrikes on Syrian military bases that killed Iranian soldiers, prompting Tehran to vow revenge. Israel has neither confirmed nor denied involvement.
Mr Netanyahu told the Israeli Cabinet on Sunday that Iran has delivered advanced weapons to Syria "in order to attack us both on the battlefield and on the home front".
"We are determined to block Iran's aggression against us even if this means a struggle. Better now than later," he said. "We do not want escalation, but we are prepared for any scenario".
Israel is widely believed to be the only nuclear-armed state in the Middle East, but has never publicly disclosed its arsenal.