South Korea Thursday criticized an "unhelpful" visit to North Korea by a senior aide to Japan's prime minister, saying it weakened the united front needed to deal with Pyongyang.

Isao Iijima arrived in Pyongyang on Tuesday on a visit that clearly surprised both Seoul and Washington, which have been working closely with Tokyo on coordinating North Korea policy.

On Thursday he met the ceremonial head of state Kim Yong-nam, Pyongyang's state media reported without saying what was discussed.

South Korean foreign ministry spokesman Cho Tai-young said it was "important" that the US and its two close allies continue to work in tandem.

"In that sense, we think that the visit by Iijima to North Korea is unhelpful," Cho said.

Glyn Davies, the US special representative on North Korea, who is in Tokyo on the third leg of an East Asia tour, attempted to move on from the surprise.

"I don't want to get into going back into recent days how this came about, whether or not certain parties were told about it," he said after telling reporters in Seoul the visit had been "news" to him.

"I think obviously we look forward to hearing from the government of Japan more details about this in coming days.

"All I can really do is to speculate and I don't want to be getting into speculating, that is not helpful."

China was positive on the subject, with foreign ministry spokesman Hong Lei telling reporters Beijing had "noted" reports of the trip. "And we hope relevant engagement can help relax the tension on the Korean Peninsula and maintain peace and stability of this region," he said.

AFP