Iran on Tuesday said the UN atomic watchdog was under "heavy political pressure" after the agency reported Tehran may have engaged in nuclear weaponisation studies more recently than previously thought.

"If... the old issue of alleged studies which has no foundation is brought up then it shows that heavy political pressure is being exerted on the agency," foreign ministry spokesman Ramin Mehmanparast said in reaction to the finding.

The IAEA chief Yuikya Amano said on Monday the agency had information that Iran may have been engaged in weaponisation studies more recently than previously thought.

But Mehmanparast reiterated Tehran's consistent stance that its nuclear programme was peaceful and acknowledged as such by the IAEA.

"The peaceful nuclear activities of the Islamic republic are completely within the framework of its rights and it has not deviated to non-peaceful means as mentioned in the latest report," the ISNA news agency quoted him as saying.

Amano said the agency received new information about possible military dimensions to Iran's atomic activities.

"In general terms, we have been collecting information from various sources at various times. Since the previous board in December, we have received some information raising further concerns," Amano said.

"I cannot specifically say up to when. But we can say there is some information that indicate the existence of activities beyond 2004," the Japanese diplomat added.

The agency has been investigating Iran's nuclear programme for eight years now, but has so far been unable to establish whether it is entirely peaceful as Iran claims or masks a covert drive to build a bomb as western powers believe.

The Japanese diplomat insisted the IAEA was not saying that Iran still had an active nuclear weapons programme but noted Tehran continues to refuse any questions on the issue, effectively blocking the long-running investigation.

"We have concerns and we want to clarify the matter," he said.