The United States sent a private warning to Tehran last month with serious concerns about Iranian research and development that could be used for the production of a nuclear weapon.

The Biden administration made the intervention in June after both US and Israeli officials detected suspicious nuclear activities by Iranian scientists in recent months. The intelligence appeared to confirm fears that Tehran was using the period around the US election to progress towards nuclear weaponisation.

According to a report from US news site Axios, which cited three unnamed US and Israeli officials, the US conveyed its concerns to Tehran weeks ago through a third country and via direct channels.

US officials told Axios that the Iranians responded with explanations for the nuclear activities and reaffirmed that there had been no change in their policy: Iran has repeatedly denied that it seeks nuclear weaponisation.

According to Axios, the Iranian response “addressed some of the concerns and somewhat eased anxieties over the Iranian research and development”.

A US official told Axios: "We do not see indications that Iran is currently undertaking the key activities that would be necessary to produce a testable nuclear device.

"We take any nuclear escalation by Iran incredibly seriously. And, as the President has made clear, we are committed to never letting Iran obtain a nuclear weapon—and we are prepared to use all elements of national power to ensure that outcome," the official added.

Last month, leaked documents revealed that one of Iran’s main nuclear facilities is undergoing a massive expansion that could triple or quadruple the country’s production of enriched uranium, providing Tehran with new options for quickly developing a mass arsenal of nuclear weapons.

The major project inside Iran’s most fortified nuclear site, the Fordow enrichment plant, would significantly accelerate Iran’s nuclear mission, according to confidential documents and analysis obtained by the Washington Post.

In March this year, US and Israeli intelligence said they believed Iranian scientists were engaging in advanced research including computer modeling and metallurgical research that could be used for nuclear weapon development.