Washington DC - The US announced new sanctions Monday against Iran over its nuclear program, despite ongoing negotiations between the two countries over the sensitive issue.

The latest sanctions target three Iranian citizens and an Iranian entity with links to Tehran's Organization of Defensive Innovation and Research, also known by its Persian acronym SPND.

"Iran continues to substantially expand its nuclear program and carry out dual-use research and development activities applicable to nuclear weapons and nuclear weapons delivery systems," Secretary of State Marco Rubio said in a statement.

He said Iran is the only country in the world without nuclear weapons that enriches uranium to 60% purity.

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That level far exceeds the 3.67% maximum set under the 2015 nuclear deal, which US President Donald Trump exited during his first term. Building a nuclear weapon requires 90% enrichment.

The US announced the new sanctions a day after a fourth round of talks with Iran concluded. No major breakthrough was announced after the talks, but both sides voiced cautious optimism.

The discussions began last month and aim to seal a new accord that would prevent Iran from having a nuclear weapon. Tehran denies seeking to build one.

The sanctions freeze any assets that the targeted people and entity may have in America and bans business dealings with them.

They are aimed at three senior officials of the nuclear program and a company called Fuya Pars Prospective Technologists.

"Fuya Pars Prospective Technologists, also known as Ideal Vacuum, is an SPND-affiliated company that has attempted to procure from foreign suppliers, as well as indigenously fabricate, equipment that could be applicable in nuclear weapons research and development," the State Department said.