President Donald Trump has angrily accused Iran of violating the spirit of the landmark 2015 nuclear deal but stopped short of ripping up the agreement.

In a speech at the White House, the US president announced a new strategy, saying the administration would seek to counter the regime’s destabilising activities and would impose additional sanctions to block its financing of terrorism.

But Mr Trump said he was not yet ready to implement a campaign pledge to pull the US out of the deal or re-impose nuclear sanctions.

Instead, he moved the issue to Congress and the other parties to the seven-nation accord, telling legislators to toughen the law that governs US participation and to fix a series of deficiencies in the agreement. Those include the expiration of several key restrictions under “sunset provisions” that begin to kick in in 2025, he said.

Mr Trump warned that without the fixes, he was minded to pull the US out of the deal and snap previously lifted sanctions back into place.

Without improvements, he said, “the agreement will be terminated”.

“It is under continuous review and our participation can be cancelled by me as president at any time,” he said.

House Speaker Paul Ryan backed Mr Trump’s decision to re-examine the seven-nation accord which he claimed was “fatally flawed”.

The Wisconsin Republican said weaknesses in the nuclear agreement would allow Iran “to pursue nuclear weapons under the guise of international legitimacy” once specific restrictions on Iran’s nuclear program expire after predetermined periods of time.

Paul RyanHouse Speaker Paul Ryan claims the accord is “fatally flawed” (J. Scott Applewhite/AP)

He warned simply enforcing a bad agreement was not sufficient.

Mr Trump’s announcement was essentially a compromise that allows him to condemn an accord that he has repeatedly denounced as the worst deal in American history. But he stopped well short of torpedoing the pact, which was negotiated over 18 months by the Obama administration, European allies and others.

Congress will now have 60 days to decide whether to put the accord’s previous sanctions back into place, modify them or do nothing. Any decision to re-impose sanctions would automatically kill America’s participation in the deal.

After his announcement Mr Trump said that if Congress doesn’t come up with satisfactory changes to the deal in a “very short” period of time, then he’s prepared to “terminate” it.

He said he was “very unhappy with Iran” and warned that the country “has to behave much differently”.

Mr Trump defended his idea for Congress to come up with a fix.

“I like the two-step process much better,” he said.

Still, he said, he may choose to pull out at a later date: “We’ll see what happens over the next short period of time.”

Ali Larijani, Iran’s parliament speaker, said that any US move against a nuclear deal with Iran would be an “insult” to the United Nations which had given the deal its blessing.

He added that any revision of the deal would allow Iran to take its own actions, and warned that the US move could destabilise the international situation.

“We will continue to adhere to our obligations … for as long as other parties observe the agreement,” he said on a visit to Russia.

American allies, who have pressed the White House to remain in the nuclear accord, were closely watching the president’s address.

The European parties to the accord — Germany, France and Britain — along with the other parties, Iran, Russia and China, have ruled out reopening the deal. But some, notably France, have signalled a willingness to tackle unresolved issues in supplementary negotiations.

Among those issues are the expiration of several restrictions on advanced nuclear activity under so-called “sunset clauses” that will allow Iran to begin ramping up its enrichment capabilities after 10 years, the end of an arms embargo and the eventual easing of demands for a halt to its missile program.

In the speech, Mr Trump hoped to “recruit” the Europeans into joining his broad strategy, particularly by punishing the Revolutionary Guard, which he and his national security team believe is fomenting instability, violence and extremism throughout the Middle East and beyond, according to one official.