The International Atomic Energy Agency declared on Thursday that Iran was not complying with its nuclear nonproliferation obligations, the first time the U.N. watchdog has passed a resolution against the country in 20 years.

The long-anticipated censure vote by the agency’s board of governors in Vienna came at a time of high tension over Tehran’s nuclear program, with American and European officials saying they believe that Israel may be preparing an imminent military strike against Iran.

Iran condemned the vote, calling it political in a joint statement from its foreign ministry and national atomic energy agency. The resolution “completely called into question the credibility and prestige” of the nuclear watchdog, they said.

The statement added that Iran will “launch a new enrichment center in a secure location and replace the first-generation machines” at another site with more modern equipment.

The I.A.E.A. also said that Iran had consistently failed to provide information about undeclared nuclear material and activities at multiple locations.

The resolution was put forward by the United States, Britain, France and Germany and passed easily, with 19 votes of the 35-nation board. Russia, China and Burkina Faso voted against, and 11 other countries abstained, while two did not vote.

Iran had reacted angrily to the prospect of the vote and threatened to leave the Nuclear Nonproliferation Treaty, which came into force in 1970. Iran is a signatory but has not ratified a section that would allow inspectors to search areas of the country where they suspect nuclear activity. But the vote was also seen as part of the diplomacy around the negotiations on Iran’s nuclear program between Washington and Tehran.

Iran’s foreign minister, Abbas Araghchi, said on Wednesday that a move to censure the country would “compel Iran to react STRONGLY. Blame will lie solely and FULLY with malign actors who shatter their own relevance.”

After the vote, Mr. Araghchi confirmed that he will attend a sixth round of talks on Sunday in Oman but cautioned that the censure added complexity to already-delicate negotiations. He will meet President Trump’s special envoy, Steve Witkoff, according to an American with knowledge of the talks.

Mr. Araghchi and others have warned that any military action against Iran by the United States or by Israel would produce severe consequences, potentially including strikes on American military bases.

Mr. Trump sought to play down Iran’s warnings. “I would love to avoid a conflict,” he told reporters at the White House on Thursday. But, he added, “they’re going to have to be willing to give us some things that they’re not willing to give us right now.”

He also acknowledged the chances of an Israeli strike on Iran but warned that such an attack could “blow it,” in reference to a U.S.-Iran deal, which he called “fairly close.”

The diplomatic effort is aimed at ensuring that Iran stops or sharply reduces uranium enrichment, which is needed for a nuclear weapon. Iran has said its nuclear program is for civilian use, not to develop weapons.

The Trump administration has proposed an arrangement that would provide Iran with fuel for reactors but prevent it from building a nuclear weapon or enriching uranium. Iran has said it will never give up the right to enrichment.

The nuclear agency vote was another indication that American and European patience is wearing thin. But the resolution did not immediately refer Iran’s noncompliance to the Security Council to consider more sanctions on Tehran. It is an effort to get Iran to comply, said a senior European diplomat who spoke on condition of anonymity to discuss private deliberations.

Any Iranian move to increase nuclear activity would probably increase Israel’s belief that a military attack is the only way to stop Tehran from obtaining a nuclear weapon. Iran already has enough highly enriched uranium at near weapons-grade to build 10 bombs in less than a year, according to the I.A.E.A.

Iran’s continued failure to comply with the nuclear watchdog’s requirements could be grounds for the Security Council to restore, or “snap back,” heavy economic sanctions on Tehran that were lifted as part of the 2015 deal that was agreed with the Obama administration, the five permanent members of the Security Council, the European Union and Germany. Mr. Trump withdrew the United States from that agreement in 2018, a move that Iran says gave it license to abandon its commitments to limit nuclear activity.

Under the 2015 deal, which expires Oct. 18, any participant can trigger the snapback sanctions, which would reinstate the multinational and U.N. sanctions lifted under the Obama-era agreement.

Those sanctions are in addition to American ones and are severe. They include an embargo on selling conventional weapons to Iran, asset freezes, banking restrictions and various restrictions on uranium enrichment and nuclear-related trade.

If those sanctions were reinstated, the restrictions would come back into force after 30 days unless the Security Council passed a resolution to continue sanctions relief.

That threat is also intended to push Iran toward a diplomatic resolution. But the snapback can only be used before the 2015 deal expires.