The US president has threatened not to renew sanction waivers that make up a key part of the deal. Tehran says it will "not accept any change" to the internationally recognized agreement.
The president of the International Atomic Energy Agency and Iran's vice president signed a clarification of the deal in 2016
The Iranian government said on Saturday that it refused to renegotiate the terms of its 2015 nuclear deal, following comments from US President Donald Trump about the deal's "terrible flaws."
"The nuclear deal is an internationally recognized and concluded pact and is not renegotiable," Tehran's Foreign Ministry said in a statement, adding that Iran "will not accept any change in the deal, neither now nor in future."
The agreement, known as the Joint Comprehensive Plan of Action (JCPOA) was signed by former US President Barack Obama and other world leaders after two years of intense talks, marking a significant shift after decades of animosity between Tehran and the West. The deal aims to prevent Iran from creating nuclear weapons while allowing the use of nuclear energy for peaceful purposes.
US regulations require the president to endorse JCPOA every 90 days and extend waivers of economic sanctions against Iran every 120 days.
Although Trump approved the waivers on Friday, he has railed against the deal since his presidential campaign, particularly against what he calls "sunset clauses" that allow Tehran to gradually resume wider nuclear activities in the next decade.
The president charged that Iran has gotten "far too much in exchange for far too little," and threatened not to renew the waivers when the next approval period comes.
Read more: Opinion: Iranians want change
Iran, Russia warn US
Despite signing off on the waivers, Trump's Treasury Department targeted several Iranian officials with sanctions over human rights abuses and ballistic missile development. Tehran has accused Washington of having "crossed all behavioral red lines of the international community," by targeting judiciary chief Sadegh Amoli Larijani.
Iran promised a strong reaction to the sanctions, which it said violate international law and prior US commitments.
Moscow, for its part, warned Washington that going back on the deal would be a grave mistake.
"This could be one of Washington's big foreign policy mistakes, a big miscalculation," said Deputy Foreign Minister Sergei Ryabkov, adding that Russia would work hard to keep the agreement alive.
es/jlw (AP, dpa)