Vienna International Centre, where IAEA offices are located
Vienna International Centre, where IAEA offices are locatediStock

The United States on Thursday threatened future action against Iran at the UN nuclear watchdog if Tehran keeps "stonewalling" the watchdog by denying it the cooperation and answers it seeks on issues including long-unexplained uranium traces, Reuters reported.

At a quarterly meeting of the International Atomic Energy Agency's (IAEA) 35-nation Board of Governors, Washington called on Iran to cooperate with IAEA inspectors who for years have been seeking explanations from Tehran on the origin of uranium particles at undeclared sites.

The IAEA has long sought answers from Iran about two sites near Tehran that inspectors say bore traces of man-made uranium.

The sites have been identified as as Turquzabad and Varamin, just outside of Tehran.

At Varamin, the IAEA in a March report said that inspectors believe Iran used the site from 1999 until 2003 as a pilot project to process uranium ore and convert it into a gas form, which then can be enriched through spinning in a centrifuge. The IAEA said buildings at the site had been demolished in 2004.

Turquzabad was identified by Prime Minister Benjamin Netanyahu during his address before the UN General Assembly in 2018 as a "secret atomic warehouse."

The IAEA said in a report in May of last year it no longer had questions about a third questioned site called Marivan near Abadeh in southern Iran.

The United States has stopped short, for now, of seeking a resolution against Iran at the IAEA over its failure to cooperate. Diplomats have cited the US presidential election in November as a reason Washington has been reluctant to do that.

"We believe we have come to the point that we and the broader international community must consider anew how to respond to Iran's continued stonewalling," the United States said in a statement to the Board meeting, according to Reuters. "We cannot allow Iran's current pattern of behavior to continue."

"It is our strongly held view that Iran's continuing lack of credible cooperation provides grounds for pursuing further Board of Governors action, including the possibility of additional resolutions and consideration of whether Iran is once again in noncompliance with its safeguards obligations," it said.

Thursday’s statement by the US came just one day after it called on Iran to dilute all of the uranium it has enriched to up to 60% purity, close to the weapons-grade level of roughly 90%.

The US call followed an IAEA report last week which said that Iran's stock of uranium enriched to up to 60% had fallen slightly in the past quarter as it had diluted, or "downblended", more of its most highly enriched material than it had produced.

The report stressed that Iran still has enough of that material, if enriched further, to fuel two nuclear weapons and enough for more bombs at lower enrichment levels.