VIENNA (Reuters) - Iran has remained within the main limits on its nuclear activity set by its 2015 deal with six world powers, the U.N. atomic watchdog said in its first report since U.S. President Donald Trump decertified Iranian compliance with the terms.

FILE PHOTO: Iran's national flags are seen on a square in Tehran February 10, 2012, a day before the anniversary of the Islamic Revolution. REUTERS/Morteza Nikoubazl/File Photo

Iran undertook to curb its uranium enrichment program in return for relief from international sanctions that crippled its economy, and U.N. nuclear inspectors have repeatedly verified Tehran’s adherence to the key aspects of the accord.

Trump has called the agreement between Iran, the United States, Britain, France, Germany, Russia, China and the European Union “the worst deal ever” and he disavowed Iran’s compliance last month. His decision did not constitute a U.S. exit from the accord but raised concern about its staying power.

Trump’s move, at odds with the commitment of the other parties to the deal, meant the U.S. Congress must decide by mid-December whether to reimpose economic sanctions lifted under the accord, reached under his predecessor Barack Obama.

If Congress reimposes the sanctions, the United States would in effect be in violation of the deal and it would likely fall apart. If lawmakers do nothing, the deal remains in place.

In response, Iranian Supreme Leader Ayatollah Ali Khamenei has said Tehran will stick to the nuclear accord as long as the other signatories respected it, but would “shred” the deal if Washington pulled out.

If the deal unravels, it would strengthen hardline opponents of Hassan Rouhani, Iran’s pragmatist president who opened up diplomatic channels to Western powers to enable nuclear diplomacy after years of worsening confrontation.

Iran’s stock of low-enriched uranium as of Nov. 5 was 96.7 kg (213.2 pounds), well below a 202.8-kg limit set by the deal, and the level of enrichment did not exceed a maximum 3.67 percent cap, said the confidential International Atomic Energy Agency report sent to IAEA member states and seen by Reuters.

Iran’s stock of so-called heavy water, a moderator used in a type of reactor that can produce plutonium, a potential nuclear bomb fuel, stood at 114.4 metric tonnes, below a 130-tonne limit agreed by the parties to the deal.

The 3.67 percent enrichment and 202-kg stockpile limit on uranium, and the 130-tonne cap on heavy water, aim to ensure that Iran does not amass enough material of sufficient fissile purity to produce a nuclear bomb. Such a device requires uranium to be refined to around 90 percent purity.

IAEA Director General Yukiya Amano told Reuters in September he would welcome clarification from the powers on how the agency should monitor Iran’s implementation of the so-called Section T of the nuclear pact that deals with certain technologies that could be used to develop an atom bomb.

Russia had been critical of the agency’s monitoring of Section T provisions, but Monday’s report said the IAEA had verified Iran’s commitment to the section.