US President Donald Trump on Friday gave the Iran nuclear deal a final reprieve but warned European allies and Congress they had to work with him to “fix the terrible flaws” of the pact or face US withdrawal.

Trump said he would waive nuclear sanctions against Iran but only as a “last chance” and would not do so again. The ultimatum puts pressure on Europeans - key backers and parties to the 2015 international agreement – to satisfy Trump, who has called the deal "the worst ever."

A senior administration official said Trump wants the deal strengthened with a follow-on agreement in 120 days or the United States will unilaterally withdraw from the international pact.

Iranian Foreign Minister Mohammad Javad Zarif responded on Twitter that the deal was not renegotiable and that Trump's stance "amounts to desperate attempts to undermine a solid multilateral agreement."

Trump, who has criticized the deal sharply both as a presidential candidate and since taking office, had privately chafed at having to once again waive sanctions on a country he sees as a rising threat in the Middle East.

"This is a last chance," Trump said in a statement. "In the absence of such an agreement, the United States will not again waive sanctions in order to stay in the Iran nuclear deal. And if at any time I judge that such an agreement is not within reach, I will withdraw from the deal immediately."

While Trump approved the sanctions waiver, the Treasury Department announced new, targeted sanctions against 14 Iranian entities and people.