The chief of the International Atomic Energy Agency (IAEA) says North Korea's nuclear ambitions are the biggest challenge to the U-N nuclear watchdog.

IAEA Director General Mohamed ElBaradei made the remarks when he met with Senior Vice Foreign Minister Ichiro Aisawa in Tokyo Thursday.

A Japanese official said ElBaradei also expressed understanding for Japan's concerns over the nuclear ambitions pursued by North Korea and Iran and Seoul's past nuclear activities.

The IAEA head reportedly stressed the need to resolve such nuclear issues through dialogue.