WASHINGTON — The Trump administration will formally demand on Thursday that the United Nations punish Iran with bruising sanctions for violating an agreement to limit its nuclear program — a deal from which the United States withdrew two years ago.
The push sets up a new confrontation at the Security Council, where European allies are resisting the so-called snapback sanctions in a last-ditch attempt to hold together the fraying 2015 accord.
“It’s a snap back. Not uncommon,” President Trump told reporters on Wednesday evening in Washington. He said he had directed Secretary of State Mike Pompeo to inform the Security Council that nearly all of the United Nations sanctions against Iran that were lifted as part of the nuclear deal would be restored.
But diplomats in Europe, China and Russia — the other powers that brokered the deal during the Obama administration — have questioned the legality of the demand because the United States is no longer part of the nuclear agreement.
Mr. Pompeo, who is set to make the administration’s case on Thursday and Friday at the United Nations headquarters in New York, noted earlier Wednesday that the nuclear accord was rooted in a Security Council resolution. As such, he said, Iran’s violations must be punished as they would under any other international commitment that is backed by the world body.
“We have every expectation that every country in the world will live up to its obligations,” Mr. Pompeo said when asked about doubts that other global powers on the Security Council would agree to reimpose sanctions on Iran’s financial institutions and industries.
“The enforcement mechanisms will be just the same enforcement mechanisms we have for all of the U.N. Security Council resolutions,” Mr. Pompeo said at a State Department news conference.
The Trump administration had for months warned that it would seek to trigger the U.N. sanctions if an arms embargo against Iran was allowed to expire in October, as scheduled under the nuclear deal. The U.S. plan to extend the embargo was decisively defeated in a Security Council vote last week, marking an embarrassing diplomatic rebuke to Washington.
The 2015 nuclear agreement sought to limit Iran’s nuclear program and start bringing the country out of economic and diplomatic isolation. Tehran had abided by the terms of the deal for more than a year after the Trump administration withdrew from it.
At the same time, the other nations that brokered the agreement — Britain, China, France, Germany and Russia, along with the European Union — insist that the United States cannot force international sanctions on an accord it is no longer recognizing.
“The U.S. cannot be considered as a J.C.P.O.A. participant,” the European Commission said in a statement, referring to the Joint Comprehensive Plan of Action, a formal name for the Iran nuclear deal.
“We therefore consider that the U.S. is not in a position to resort to mechanisms reserved for J.C.P.O.A. participants,” including the sanctions’ snapback, the statement said.
The American demand seeks to fulfill a campaign promise by Mr. Trump four years ago to dismantle the nuclear deal, which was one of President Barack Obama’s signature diplomatic achievements. Instead, the Trump administration has sought to force Iran to negotiate a new, broader accord that also curbs Tehran’s military activities and support for Shiite militias stirring unrest in the Middle East.
Without European support, it is not clear how the United States alone would enforce the U.N. sanctions, although most global trade is routed through American financial institutions. The new sanctions would also extend the arms embargo on Iran, overriding the Security Council’s rejection last week.
It is also unknown if Iran would declare the nuclear deal defunct if the Security Council refuses to reimpose the sanctions. Most analysts believe that world powers and Iran alike are trying to delay action until after American elections in November to see if Mr. Trump remains in office or whether a new course would be charted by the Democratic nominee, Joseph R. Biden Jr.
Senator Chris Van Hollen, Democrat of Maryland, hinted at that strategy on Wednesday morning during a forum hosted by the European Union’s diplomatic missions in Washington.
Noting the “successful effort” that brought Iran to the negotiating table during the Obama administration, Mr. Van Hollen said sanctions “can more effectively bring pressure to achieve our goals if we do them in a coordinated and united way — the E.U. and the United States using our economic leverage.”
“Clearly you have a multiplier effect if both of these important entities are working together,” Mr. Van Hollen said.