Iran has begun installing advanced centrifuge machines for enriching uranium at its nuclear plant at Natanz, says the UN's nuclear watchdog.
The US said if confirmed it would be "another provocative step".
International talks over Iran's nuclear programme are due to resume in Kazakhstan next week.
Western powers fear Tehran is seeking weapons technology, but Tehran says it is refining uranium only for peaceful energy purposes.
The Natanz facility, in central Iran, is at the heart of the country's dispute with the UN's watchdog.
The IAEA released a report each quarter detailing its progress at monitoring Iran's nuclear development.
The BBC obtained a copy of the latest report, which has not yet been officially released.
It concludes: "The director general is unable to report any progress on the clarification of outstanding issues including those relating to possible military dimensions to Iran's nuclear programme."
It adds that despite intensified dialogue with Iran, no progress has been made on how to clear up the questions about Iran's nuclear work.
The IAEA has made similar complaints in previous quarterly reports, and Iran is under an array of sanctions as a result of its lack of co-operation.
Iran had informed the IAEA in a letter on 23 January that it planned to introduce a new model of centrifuge called the IR2m, which can enrich two or three times faster than current equipment.
Gas centrifuges are used to increase the proportion of fissile uranium-235 atoms within uranium.
For uranium to work in a nuclear reactor it must be enriched to contain 2-3% uranium-235 while weapons-grade uranium must contain 90% or more uranium-235.
Israeli Prime Minister Benjamin Netanyahu said the new centrifuges could cut by a third the time Iran, one of Israel's fiercest opponents in the Middle East, needed to create a nuclear bomb.
US state department spokeswoman Victoria Nuland said the report development at Natanz was "not surprising".
"The installation of new advanced centrifuges would be a further escalation, and a continuing violation of Iran's obligations under the relevant UN Security Council resolutions and IAEA board resolutions," she said.
But she added that Iran had the opportunity to allay the international community's concerns during talks in Kazakhstan next week.
Starting on 26 February, the talks will involve Iranian officials, the five permanent members the UN Security Council, and Germany.