South Korea Thursday criticised 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," Hong said.

Japanese Prime Minister Shinzo Abe has refused to comment on the purpose of Iijima's trip.

But Abe said that he himself would consider meeting North Korean leader Kim Jong-Un if it could help resolve the longstanding issue of Pyongyang's kidnapping of Japanese citizens.

The North's state media said Iijima held talks Wednesday with Kim Yong-Il, secretary of the ruling Workers' Party Central Committee.

Iijima was also a senior aide to Japan's then-prime minister Junichiro Koizumi, and is known to have played a role in organising his trips to Pyongyang in 2002 and 2004 for talks with then-North Korean leader Kim Jong-Il.

His visit fuelled speculation that the North may be trying to thaw icy relations with Japan at a time when ties with the United States and South Korea have gone into deep freeze after nuclear and missile tests.

The US, along with its two Asian allies, has increased pressure on Pyongyang to drop its nuclear ambitions and to join the international community.

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