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 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.