“Iran is about to reduce its compliance with the J.C.P.O.A., but not formally withdraw,” said Cliff Kupchan, the chairman of the Eurasia Group, a political risk consultancy. “That’s a distinction with a big difference.”

While Iran may increase its stockpile of enriched uranium and possibly test advanced centrifuges, Mr. Kupchan said, “I doubt they’ll take drastic steps in these areas, and Iran will continue to comply with other important aspects of the J.C.P.O.A.”

Relations between Iran and the United States have worsened substantially under the Trump administration, which has imposed onerous sanctions on the Iranians since the president renounced the nuclear agreement on May 8, 2018.

The administration has sought to drive exports of Iranian oil to near zero and has designated the Islamic Revolutionary Guards Corps as a terrorist organization, among other actions. The Iranians have responded so far by designating the United States Central Command as a terrorist organization, suggesting that American forces posted in other countries are at risk from Iranian attack.

On Sunday the administration said it was strengthening American military deployment in the Persian Gulf in response to what it called an increased Iranian threat.

The risk of an armed confrontation between Iran and the United States figures prominently in a statement signed by a bipartisan group of more than 60 national security experts, exhorting the administration to rejoin the nuclear agreement. The group, known as the National Coalition to Prevent an Iranian Nuclear Weapon, had planned to release the statement on the anniversary of the American withdrawal.

“The U.S. unilateral withdrawal from the J.C.P.O.A. on May 8, 2018 significantly undermined America’s national security and has fueled momentum for a possible new conflict in the Middle East,” the signers wrote. “If Iran also withdraws from the J.C.P.O.A., or from the Nuclear Nonproliferation Treaty (N.P.T.), the trans-Atlantic alliance will be dealt a severe blow and a Middle Eastern nuclear arms race could ensue.”

The group has long opposed Mr. Trump’s position on the accord and had urged him to abide by it in the months before he scrapped the agreement.