Sanctioning 14 individuals and 17 entities, US officials acknowledged that the nuclear work was in the past but said Washington wanted to make the targeted figures "radioactive."

Secretary of State Mike Pompeo on Twitter called the sanctions part of the "maximum pressure campaign" on Iran as the United States tries to roll back the clerical regime's regional influence.

"We'll be relentless in denying Iran the ability to engage in WMD proliferation and all its outlaw activities," said Pompeo, who is on a Middle East tour to build a united front against Iran.

President Donald Trump last year pulled out of an international accord negotiated under his predecessor Barack Obama, under which Iran drastically scaled back its nuclear program, and instead imposed sweeping sanctions.

But European nations still back the accord and the International Atomic Energy Agency has repeatedly said that Iran remains in compliance.

But the US Treasury Department said it was alarmed over the continued existence of the Organization of Defensive Innovation and Research, whose Farsi acronym is SPND, saying that it could get back to work -- including after some prohibitions under the nuclear deal start running out in 2025.

A senior US official, speaking to reporters on condition of anonymity, said that the United States wanted to "continue to stigmatize SPND and the reconstitution program-in-waiting that it represents."

The sanctions aim to "make it as unattractive as possible to be a part of that organization, make it hard to recruit the next generation of illicit nuclear weapons scientists and to make it all more clear that this is an option that is not and cannot be allowed to be made available to Iran," he said.

The sanctioned individuals include people who work with the Shahid Karimi Group, which the Treasury Department said focused on missile and explosives projects for SPND, and the Shahid Chamran Group, which researches electromagnetics and wave generation.

The US official said that the SPND was still in place and headed by Mohsen Fakhrizadeh, a physicist identified by US and Israeli intelligence as the aspiring father of Iran's nuclear bomb.

"It's as if some evil version of Robert Oppenheimer had been kept in charge of keeping the Manhattan Project crew together years afterward," the official said, referring to the founder of the US nuclear program in World War II.