Britain, France and Germany have expressed grave concern about the detection of previously undeclared nuclear material at two sites in Iran.

The International Atomic Energy Agency's Board of Governors discussed Iran's nuclear programs in meetings in Vienna that ended on Thursday.

The governors discussed the presence of previously undeclared uranium particles at three sites in Iran.

Iran has explained that nuclear material may have been used in a research facility related to a former mine at one of the sites.

IAEA Director General Rafael Grossi told a news conference on Monday that the agency received "a reply that is plausible." But he added, "We do not have any means to actually prove or disprove" Iran's explanation.

Britain, France and Germany said in a joint statement on Wednesday that the agency reports "no progress towards resolving the remaining outstanding safeguards issues" at the other two sites.

The statement urged Iran to clarify and resolve all outstanding issues before the next meeting of the IAEA Board of Governors, scheduled in September. It said that if Iran fails to do this, "the Board will have to be prepared to take further action."