Washington, May 13 : The US has imposed sanctions on three Iranian nationals and one Iranian entity with ties to Tehran's Organisation of Defensive Innovation and Research, which is known by its Persian acronym, SPND.

The SPND is the direct successor organisation to Iran's pre-2004 nuclear weapons program, also referred to as the Amad Project.

The US Secretary of State, Marco Rubio, in a press statement on Monday, stated that "all individuals sanctioned are involved in activities that materially contribute to, or pose a risk of materially contributing to, the proliferation of weapons of mass destruction."

"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. Iran is the only country in the world without nuclear weapons that is producing uranium enriched to 60 per cent, and it continues to use front companies and procurement agents to obscure its efforts to acquire dual-use items from foreign suppliers," the statement said.

"The United States' actions are intended to delay and degrade the ability of SPND to conduct nuclear weapons research and development. Today's actions demonstrate the United States' commitment to ensuring that Iran never obtains a nuclear weapon," it further added.

The remarks followed after the US and Iran on Sunday concluded the fourth round of indirect nuclear talks in Oman's capital, Muscat.

"The fourth round of indirect Iran-US negotiations is concluded; difficult but useful talks to better understand each other's positions and to find reasonable & realistic ways to address the differences. Next round will be coordinated and announced by Oman," Esmaeil Baqaei, Iranian Foreign Ministry spokesperson, posted on X.

Iranian Foreign Minister Seyed Abbas Araghchi said that indirect negotiations with the United States in Oman to revive the 2015 nuclear deal had become "much more serious and frank," while Iranian President Masoud Pezeshkian rejected US demands for Tehran to dismantle its nuclear infrastructure.

Araghchi, speaking to Iran's state-run IRIB TV after the fourth round of negotiations in Oman's capital, said the discussions had shifted from general topics to more specific proposals.

He characterised the talks as "forward-moving" but acknowledged the growing complexity of the issues.

Both sides agreed to continue the discussions.

Meanwhile, President Pezeshkian firmly rejected US calls to dismantle Iran's nuclear infrastructure.

"This is unacceptable. Iran will not relinquish its peaceful nuclear rights," he declared, reaffirming Tehran's stance that its nuclear program is for civilian purposes.

/IANS