North Korea's nuclear programme is a matter of "serious concern," the UN atomic agency IAEA said in a report released Thursday, referring in particular to two sites Pyongyang has stopped it from visiting.

"The nuclear programme of (North Korea) is a matter of serious concern," said the report from the International Atomic Energy Agency.

North Korea's statements about uranium enrichment activities and the construction of a light-water reactor "continue to be deeply troubling", the report added.

The country had in recent months made significant progress in building this reactor, the report said.

In late February, the United States announced it had agreed a moratorium with North Korea on nuclear activities and ballistic missile tests, in exchange for a resumption of food aid to the hermetic Stalinist state.

In March, North Korea invited the IAEA to send inspectors to help implement the deal -- a significant rapprochement given it expelled agency inspectors in 2009 and has since refused to cooperate with the agency.

But after a much-vaunted missile launch by Pyongyang on April 13, Washington suspended the deal, even though the launch failed.

Talks between the IAEA and North Korea have thus remained on hold.

Nevertheless, Thursday's report from the agency called on North Korea to renew its cooperation with them.

"The Agency will continue to maintain its readiness to play an essential role in verifying (North Korea's) nuclear programme," it added.

The North has staged two nuclear tests, in 2006 and 2009. In November 2010, it unveiled its uranium enrichment factor. Until now, it has used plutonium in its nuclear devices.