Diplomats at the United Nations say Washington will face tough challenge at the Security Council if it pushes for extending the arms embargo against Tehran in relation to the historical nuclear deal Washington left in 2018.

The agreement — officially called the Joint Comprehensive Plan of Action (JCPOA) — was signed in 2015 between Iran and six major world states and endorsed by UNSC Resolution 2231.

However, the US unilaterally withdrew from what President Donald Trump called “the worst deal ever” in May 2018 and re-imposed anti-Iran sanctions. It has also been coercing the remaining signatories to the JCPOA to follow suit.

Under UNSC Resolution 2231, the UN arms embargo on Iran — in place since 2006/2007 — will be lifted in October 2020. The US has repeatedly expressed its anger at the possible termination of restrictions on Iran’s import and export of arms.

On Tuesday, diplomats told Reuters that Washington is planning to use a threat to trigger a return of all UN sanctions against Iran as leverage to get the Security Council to prolong the arms embargo on Tehran.

A US official, who was speaking on condition of anonymity, said Washington has shared its strategy with Britain, France and Germany, who are permanent Security Council member states and parties to the Iran deal.

The diplomats said the scheme has not been shared with the remaining 11 council members, including veto-wielders Russia and China, the two other signatories to the JCPOA that are deemed certain to oppose the arms embargo on Iran.

Reports say if the Security Council refuses to revive the embargo, the US will try to trigger a so-called snapback of all UN sanctions on Iran by claiming that it is still a party to the JCPOA and that Iran is in significant violation of the nuclear pact.

The New York Times said on Sunday that US Secretary of State Mike Pompeo was laying the groundwork to present a legal argument to the UN that Washington remains a “participant state” in the JCPOA.

It is “part of an intricate strategy to pressure the United Nations Security Council to extend an arms embargo on Tehran,” the report said.

Reuters, however, cited the UN diplomats as saying that Washington’s plan will likely be challenged since the US is no longer a party to the deal.

“It will be dead on arrival,” a Security Council diplomat predicted.

“It’s very difficult to present yourself as a compliance watcher of a resolution you decided to pull out of,” a European diplomat said. “Either you’re in or either you’re out.”

Another European official said, “It’s going to be messy from a Security Council standpoint because, regardless of what (Britain, Germany and France) think, Russia and China are not going to sign up to that legal interpretation.”

Iranian Foreign Minister Mohammad Javad Zarif has dismissed Pompeo’s reported scheme, saying the plan to return to the JCPOA is rooted in the failure of Washington’s “maximum pressure” campaign against the Iranian nation.

Meanwhile, a Western diplomat, speaking on condition of anonymity, told  AFP, “You cannot cherry-pick a resolution saying you implement only parts of it but you won’t do it for the rest.”

Similarly, Kelsey Davenport, director for non-proliferation policy at the US-based Arms Control Association, said “if Pompeo goes through with this plan, snapping back sanctions on Iran collapses the JCPOA.”

Even more significant, she added, the US measure could lead Iran to make good on threats to exit the nuclear Non-Proliferation Treaty (NPT).

“This is just another step that would undermine US credibility, make future negotiations with Iran more difficult and increase the risk of a nuclear crisis in the region,” she said.

Russia also slammed the US plan, with Permanent Representative to International Organizations in Vienna Mikhail Ulyanov tweeting, “The country which officially ceased its participation in the #IranDeal cannot remain its participant by definition.”

The Russian Mission Vienna also said in a post on Twitter, “May 8, 2018 the #US says it “ends” and “terminates” its participation in the #JCPOA. Now there are claims that it still considers itself a participant. Is this a “Catch-22” sort of situation for Iran, UNSC and the whole world?”