"The UK reserves itself the right to call for a special session of the (IAEA) board," Peter Jenkins told reporters.

"It is to indicate that there might be circumstances where we feel the time has to come to report in Iran to the Security Council," he said.

The United States charges that Iran is using a civilian nuclear programme to hide covert development of atomic weapons, something Tehran denies.

In September, the IAEA board found Iran in contravention of the nuclear Non-Proliferation Treaty (NPT) for hiding sensitive nuclear activities, a finding that requires notifying the Security Council, which can impose sanctions.

The IAEA board met again on Thursday. According to an official statement summarising the meeting in Vienna and obtained by AFP, it postponed referring Iran to the UN Security Council.

Earlier in the day, Jenkins' US counterpart at the International Atomic Energy Agency issued a similar caution to Tehran.

Washington backed a call to hold off taking Iran before the Security Council, US IAEA ambassador Gregory Schulte said, but warned that referral would happen soon if Iran did not meet its non-proliferation obligations.