US President Joe Biden's administration sent a private message to Iran about it against recent nuclear developments, amid fears the country is attempting to weaponise its programme.
The warning was sent to Tehran last month highlighting US-Israeli knowledge of new research activities by Iranian scientists which could indicate steps are being taken towards nuclear weapons, Axios reported on Wednesday.
US and Israeli officials were reportedly investigating the changes to Iran's nuclear programme to understand whether this reflects a shift in policy by Supreme Leader Ayatollah Ali Khamenei.
Iran insists its nuclear programme is solely aimed at generating energy for the country's rapidly-growing population, but many countries in the region suspect this is being used as cover for the pursuit of nuclear weapons.
"We do not see indications that Iran is currently undertaking the key activities that would be necessary to produce a testable nuclear device," a US official said, according to Axios.
"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."
Tehran responded, insisting its activities are purely peaceful and there is no change in policy.
Although the exchange eased some Israeli and US suspicions about the recent activities, general questions remain about Tehran's nuclear programme, an official told the website.
There are fears Iran's building of a bomb would spark nuclear proliferation in the region, with Saudi Crown Prince Mohammed bin Salman saying Riyadh would do the same if Tehran became a nuclear-armed power.
Saudi Arabia is also seeking to develop civilian nuclear energy in the kingdom, but there are reservations in Washington this could be directed toward military purposes at a later date.
Washington withdrew from a nuclear agreement with Iran under President Donald Trump in 2018 with harsh sanctions enacted on Tehran, with no sign of the US rejoining the Joint Comprehensive Plan of Action (JCPOA) under Biden.
Israel already has nukes, although it has never publicly admitted to this and the size of its arsenal is still largely unknown.