The United Nations' nuclear watchdog has detected uranium particles at an undeclared site in Iran, it said in its latest report on the country's nuclear programme on Monday.
The report from the International Atomic Energy Agency (IAEA), seen by AFP, also confirmed that Iran has ramped up uranium enrichment, in breach of a landmark 2015 deal with international powers.
Britain, France, Germany and the EU said after a meeting in Paris to discuss the situation that they were "extremely concerned" by the latest developments.
The report said the IAEA's inspectors had "detected natural uranium particles of anthropogenic origin at a location in Iran not declared to the agency."
The particles are understood to be the product of uranium which has been mined and undergone initial processing, but not enriched.
The IAEA added that it was "essential for Iran to continue interactions with the agency to resolve the matter as soon as possible."
While the IAEA itself has not named the site in question, diplomatic sources have previously said the agency has been posing questions to Iran relating to a site where Israel has alleged secret atomic activity in the past.
Sources say the IAEA took samples from the site in the Turquzabad district of Tehran in the spring and that Iran has been slow in providing answers to explain the test results.
- Uranium stockpile -
On Iran's uranium enrichment activities, the IAEA report said Tehran's stockpile had now reached the equivalent of 551 kilogrammes, as opposed to the 300-kilogramme limit laid down in the 2015 Joint Comprehensive Plan of Action.
Under that deal, Tehran agreed to curb some of its nuclear activities in return for a lifting of economic sanctions.
But the Islamic republic has said it no longer feels bound by the JCPoA after the US unilaterally pulled out of the accord last year and has reimposed sanctions.
A Vienna-based diplomat said the rate of production of enriched uranium had gone up substantially to more than 100 kilogrammes a month, and could increase further.
According to the IAEA, Iran is now enriching uranium at its Fordow facility, a development which "the Foreign Ministers of France, Germany and the United Kingdom and the High Representative of the European Union are extremely concerned" about, the western powers said.
The development "represents a regrettable acceleration of Iran's disengagement from commitments under the JCPoA, including exceeding the maximum allowed low enriched uranium stockpile and the maximum allowed enrichment limits," they said in a joint statement.
The IAEA report made no mention of an incident involving one of the IAEA's inspectors two weeks ago which led to Iran cancelling her accreditation.
- Security incident -
Iran said that the inspector triggered a security check at the entrance gate to the Natanz enrichment plant.
Without going into specifics, the IAEA has disputed the Iranian account of the incident and said last week that the inspector was briefly prevented from leaving the country, adding that her treatment was "not acceptable."
However, a diplomat described the incident as an "isolated case" and there is no indication that the IAEA's access for inspections has been restricted more broadly.
The JCPoA has been in increasing danger of falling apart since US President Donald Trump decided to pull out of the deal.
Since May, Iran has progressively breached limits under the agreement, while insisting the steps are reversible if the other parties to the deal provide relief to mitigate the effect of US sanctions.
However, despite Iran saying last week that it was now enriching uranium to five percent, the report said the highest level observed was 4.5 percent, still higher than the 3.67 laid down in the 2015 deal.
Ahead of the meeting in Paris on Monday of the British, French, German and EU officials, one European source had said that "the window of opportunity for de-escalation is narrowing very seriously."
The source expressed concern that the so-called "breakout time" needed for Iran to gain the fissile material had been kept to at least 12 months while the Iranians were abiding by the terms of the JCPoA, but that it was not starting to "come down seriously".
Iran has always insisted that its nuclear programme is exclusively peaceful and that acquiring nuclear weapons would be contrary to Islamic principles.