VIENNA — Western powers are preparing to push the UN nuclear watchdog’s board at its next quarterly meeting to declare Iran in breach of its non-proliferation obligations for the first time in almost 20 years, a move bound to enrage Tehran, diplomats said.

The step is likely to further complicate talks between the United States and Iran aimed at imposing fresh restrictions on Iran’s rapidly advancing nuclear program.

Washington and its European allies Britain, France and Germany, known as the E3, proposed past resolutions adopted by the International Atomic Energy Agency’s 35-nation Board of Governors calling on Iran to quickly take steps such as explain uranium traces the IAEA found at undeclared sites.

The IAEA is preparing to send member states its quarterly reports on Iran before the next board meeting, which begins on June 9. One of those will be a longer, “comprehensive” account of issues including Iran’s cooperation, as demanded by a board resolution in November, and diplomats expect it to be damning.

“We expect the comprehensive report to be tough, but there were already no doubts over Iran not keeping its non-proliferation commitments,” one European official said.

Once that report is issued, the United States will draft a proposed resolution text declaring Iran in breach of its so-called safeguards obligations, three diplomats said. A fourth said the Western powers were preparing a draft resolution without going into specifics.

The text will be discussed with countries on the board in the coming days before being formally submitted to the board by the four Western powers during the quarterly meeting, as has happened with previous resolutions, diplomats said.

The last time the board took the step of formally declaring Iran in breach of its safeguards obligations was in September 2005 as part of a diplomatic standoff that stemmed from the discovery of clandestine nuclear activities in Iran.

The United States and IAEA now believe Iran had a secret, coordinated nuclear weapons program that it halted in 2003. Iran denies ever having had a weapons program and says it is only using nuclear technology for peaceful purposes.

A separate IAEA board resolution passed in February 2006 referred Iran’s non-compliance to the UN Security Council, which later imposed sanctions on Iran.

The diplomats said it had not yet been determined at what point the Western powers would seek to have the matter referred to the Security Council, and it is unclear what action, if any, the Security Council would then take against Iran.

The most immediate effect of a resolution is likely to be on Tehran’s talks with the United States and any further nuclear steps Iran decides to take on the ground.

A senior Iranian official told Reuters that Tehran would react to a resolution by “expanding nuclear work based on (the content of) the resolution.”

The board has passed all recent resolutions proposed by the Western powers on Iran, and there is little doubt that this one would go through as well. The only question is how large the majority would be. Russia and China have been the only countries to consistently oppose such resolutions.

Iran bristles at resolutions and other criticism of it at the IAEA board, taking steps such as accelerating and expanding its uranium enrichment program or barring top IAEA inspectors.

It is already enriching uranium to up to 60% purity, which can easily be further enriched to the roughly 90% of weapons grade. It has enough material at that level, if enriched further, for six nuclear weapons, according to an IAEA yardstick.

Meanwhile, the semi-official Fars News Agency cited an unnamed Iranian official who warned that US President Donald Trump’s threat to destroy Iran’s nuclear facilities is a clear red line and will have severe consequences.

“If the US seeks a diplomatic solution, it must abandon the language of threats and sanctions,” an unnamed Iranian official said, adding that such threats “are open hostility against Iran’s national interests.”

Trump told reporters on Wednesday at the White House: “I want [the nuclear agreement] very strong, where we can go in with inspectors, we can take whatever we want, we can blow up whatever we want, but nobody getting killed. We can blow up a lab, but nobody is gonna be in a lab, as opposed to everybody being in the lab and blowing it up.”

Trump has repeatedly threatened to bomb Iran’s nuclear facilities if diplomacy fails to resolve a decades-long dispute over Tehran’s nuclear program.

Trump said on Friday that an Iran deal was possible in the “not-too-distant future.”