Iran has launched a speedy clean-up operation at a nuclear-related site in northern Tehran hit by Israeli airstrikes that will likely remove evidence of any nuclear weapons development work, a research group said on Wednesday.

Satellite imagery "shows a significant effort by Iran to rapidly demolish damaged or destroyed buildings, likely to sanitize any incriminating nuclear weapons research and development activities," the Institute for Science and International Security said.

The institute is an independent research group that focuses on stopping the spread of nuclear weapons and is headed by David Albright, a former UN nuclear inspector.

Iran's embassy to the United Nations did not immediately respond to a request for comment. Tehran denies seeking nuclear weapons and repeatedly has said its program is for peaceful purposes.

The report on the Mojdeh site comes as the UN nuclear watchdog holds ongoing talks in Tehran on restarting inspections disrupted by the June 13-24 war between Israel and Iran and the June 22 US strikes on the country's three main nuclear facilities.

Britain, France and Germany are likely to begin on Thursday the process of re-imposing UN sanctions on Iran for violating a 2015 nuclear deal designed to prevent it from developing nuclear weapons, according to four diplomats.

Rafael Grossi, the head of the International Atomic Energy Agency (IAEA), told reporters on Wednesday at the end of a two-day visit to Washington that Iran is legally obligated to allow inspections to resume and that they should begin "as soon as possible.

The agency, he said, wants to visit "all of the sites that are relevant," including the main nuclear facilities - Fordow, Natanz and Isfahan - hit by the US and to account for Iran's stock of more than 400 kg (882 pounds) of uranium enriched to near weapons-grade purity.

MOJDEH STRIKES

Israel twice bombed the Mojdeh site, also known as Lavisan II, adjacent to Malek Ashtar University on June 18, the institute's report said, during the operation that hit hundreds of targets across Iran.

It noted that the IAEA had established a direct link between Mojdeh and the AMAD Plan, a nuclear weapons development program that the agency and US intelligence separately concluded ended in 2003.

The first Israeli strike on Mojdeh hit several buildings, the report said.

One was associated with the Institute of Applied Physics and another had a suspected link to the Shahid Karimi Group, which the US has sanctioned for work on missiles and explosive-related projects.

The group belongs to the Organization of Defensive Innovation and Research, which the US and the IAEA say is the direct successor to the AMAD Plan.

The second Israeli strike destroyed the Institute of Applied Physics building, damaged a security building and destroyed a workshop, according to a June 20 satellite image from Maxar Technologies, the report said.

A July 3 image depicted the beginning of the clean-up and removal of debris, the report said. August 19 imagery showed that the Applied Physics building and workshop had been razed and the debris completely cleared, as was the building suspected of housing the Shahid Karimi group.

"The rapid work by Iran to quickly demolish and clear the rubble of these important buildings appears to be an effort to sanitize the site and limit the availability of any possible future inspection from obtaining" evidence of nuclear weapons-related work, the report said.