Japan did not inform South Korea or the United States of an "unexpected" visit by an adviser to Japanese Prime Minister Shinzo Abe to North Korea, a Seoul government source said Wednesday, raising concerns that the trip may undermine efforts in forging a coordinated approach toward the belligerent country.

North Korea's state media confirmed that a Japanese delegation led by Isao Iijima flew to Pyongyang earlier Tuesday. Japan's Kyodo news agency had also reported on lijima's trip to North Korea, without giving any details, including its purpose.

The trip to North Korea has spawned speculation that Japan may be trying to mend frosty ties with the North amid a deepening territorial dispute with China. "The Japanese delegation's visit to North Korea was an unexpected act that is not in line with a resolute response by the international community to resolve North Korea's nuclear issue," the source said on the condition of anonymity.

"There will be some discussions on the visit among South Korea, the U.S. and Japan, which have pursued a coordinated approach toward the North's nuclear issue," the source said.

Iijima's trip to the communist country came as the top U.S. envoy on North Korea policy, Glyn Davies, is on a visit to South Korea as part of his three-nation trip that includes China and Japan.

Asked about the Japanese delegation's visit to Pyongyang, Davies replied in Seoul on Tuesday, "I had not heard of that, so that will obviously be something that I will discuss with the Japanese when I have a chance to talk to my counterparts there."

Japan normalized relations with South Korea in 1965, but has no formal ties with North Korea. Their relations remain frosty amid ongoing disputes over a dozen Japanese nationals th North has admitted to kidnapping decades ago.

Iijima's trip comes during heightened tension in the region over the North's nuclear and missile programs. Japan is strongly opposed to the communist country's pursuit of weapons of mass destruction.

Iijima's mission in North Korea is unclear but he is said to have helped arrange former Japanese Prime Minister Junichiro Koizumi's trips to Pyongyang for talks with then North Korean leader Kim Jong-il in 2002 and 2004.

During Koizumi's 2002 trip, Kim admitted that North Korean agents had kidnapped a total of 13 Japanese citizens in the 1970s and 1980s to use them as language instructors for communist spies.

The North Korean leader, who died in late 2011, then allowed eight of those Japanese abductees to return home, saying that the other five had died of infectious diseases or were killed in traffic accidents. Japan discredited the North's explanations, claiming that some of the other five may still be alive.

Since the death of Kim Jong-il, his third son Kim Jong-un has ruled North Korea. (Yonhap News)