PARIS (Reuters) - Iran risks triggering a new crisis if it does not cooperate with the U.N. nuclear watchdog after failing to answer its questions about past nuclear activities at three sites and denying it access to two of them, its chief said on Tuesday. 

Rafael Grossi, who took up his post in December, spoke to Reuters in an interview hours after he and the International Atomic Energy Agency (IAEA) he heads released a report admonishing Iran.

"We have been requesting some information and access from Iran but we haven't been getting the information we require," he said. "We have insisted and despite all our efforts we have not been able to get that, so the situation requires on my part such a step because what this means is that Iran is curtailing the ability of the agency to do its work."

Speaking after meetings in Paris, Grossi said the issue was serious and he hoped that Iran would return to full compliance after the IAEA's board of governors meets in Vienna next week.

"I sincerely hope that Iran will listen to us and listen to the voice of the international community at the board of governors and assess that it is in their own interest to cooperate with us," he said.

"We will be walking towards a crisis (if not)," he said.