The International Atomic Energy Agency (IAEA) on Thursday voiced concern over uncertainties surrounding Iran's nuclear program and criticized Tehran for barring the body's officials from accessing nuclear sites.

"The longer the current situation persists, the greater such uncertainty becomes," the UN watchdog said in a new report.

The IAEA also said it was still waiting for an explanation from Tehran about the origin of undeclared uranium particles that were detected at three locations.

The issue has been a key sticking point in wider talks between Iran and world powers seeking to revive a 2015 deal to curb Tehran's nuclear program.

While Iran has demanded the IAEA end its investigation into the matter, the UN agency says it needs answers because it has to be able to account for all nuclear material.

IAEA chief Rafael Grossi "is seriously concerned that there has still been no progress in clarifying and resolving the outstanding safeguards issues," the watchdog's report said.

It added that senior officials would conduct a technical visit to Tehran before the end of the month and expected to start receiving "credible explanations," including "access to locations and material."

The UN watchdog has repeatedly warned that it has not been able to properly monitor Iran's nuclear program since early 2021, when the Islamic Republic started to restrict the agency's access.

What happened with the 2015 nuclear accord?

The deal struck with Britain, France, Germany, China, the US and Russia meant Iran had to curb its atomic program in return for relief from Western sanctions.

The agreement collapsed in 2018 when then US President Donald Trump decided to withdraw and reimpose sanctions.

Negotiations geared towards reviving the deal began in April but have been stalled for months. 

What is the status of Iran's nuclear program?

Iran insists its nuclear program is purely for peaceful purposes and that it has no intention of making a nuclear bomb. However, it has continued enriching uranium to levels well above the 3.67% limit agreed to under the 2015 deal.

In a separate report, also released Thursday, the IAEA said Iran's stock of enriched uranium had decreased by 267.2 kg. Its stockpile now stands at 3,673.7 kg — well beyond the 202.8 kg allowed under the deal.

The shrinking stockpile can be largely put down to Iran's growing amount of highly enriched uranium, which requires more material.

Its stock of uranium enriched to 60% purity — close to the 90% required for a weapon — is now at 62.3 kg, up from 55.6 kg, the IAEA said. According to experts, 50 kg is enough to make a nuclear bomb.

Iran claims to have hypersonic missiles

The IAEA's reports were released as an Iranian general claimed — without providing evidence — that his country had developed hypersonic missiles capable of penetrating all defense systems.

Hypersonic missiles are maneuverable and can fly at more than five times the speed of sound. Like traditional ballistic missiles, they can also carry nuclear weapons.

"This system is very, very fast, and is capable of maneuvering both inside and beyond the atmosphere," General Amir Ali Hajizadeh, the commander of the Islamic Revolutionary Guard Corps aerospace unit, said in a speech on Thursday.

The Guard's missile program is not known to have hypersonic weapons in its arsenal.

IAEA chief Grossi told the AFP news agency that the claim served to "increase the attention, increase the concerns, increase the public attention to the Iranian nuclear program."

But he said he did not see it as "having any influence" on current negotiations over Iran's nuclear program.

nm/aw (Reuters, AP, AFP, dpa)