TOKYO (REUTERS) - A US envoy for North Korea sidestepped questions on Thursday on the nature of a surprise visit to Pyongyang by an aide to Japan's prime minister, but said all sides tackling North Korea's nuclear ambitions should coordinate closely.

Mr Isao Iijima's trip to the North Korean capital, Pyongyang, and his talks with senior officials there have irritated South Korea and analysts said possibly the United States as well, although China has been more positive about it.

Mr Iijima, a close aide to Prime Minister Shinzo Abe, went to the North on Tuesday, several weeks after tension subsided in the region.

North Korea had for weeks said it was on the verge of nuclear war with the South and the United States after the United Nations imposed new sanctions in response to North Korea's third nuclear test.

"This mission ... is still underway," Mr Glyn Davies, US special representative for North Korea policy, told reporters, referring to Mr Iijima's visit.

"I think we have some days to wait, all of us, before we know if there are any results from this mission," Mr Davies said.

"We all have fundamental security interests in dealing with North Korea ... It is important that we stay connected very closely." D

Mr Davies spoke after meeting Shinsuke Sugiyama, a senior Japanese foreign ministry official. He said he had received no advance notice of Mr Iijima's trip.

South Korea bluntly expressed its irritation at the trip.

"We don't think Isao Iijima's visit to North Korea was helpful," said South Korean foreign ministry spokesman Cho Tae Young said.

China said it hoped the visit would help ease tension.

According to North Korean state media, Mr Iijima has met the titular head of state, Kim Yong Nam, and the ruling Workers'Party's top official in international relations, Kim Yong Il.

Mr Abe and his top spokesman have been silent about the purpose of the visit, but experts said it was probably aimed at scoring diplomatic points ahead of an upper house election in July.

Mr Abe's Liberal Democratic Party (LDP) is expected to win the poll but it wants a huge victory to cement its grip on power.

"(Abe) wants to achieve some movement in relations with North Korea ahead of the upper house election. I don't think there will be any huge advance, though," said Masao Okonogi, a North Korea expert at Keio University in Tokyo.

A confidential report by a UN panel of experts, seen this week by Reuters, said the tougher sanctions had significantly delayed expansion of the North's nuclear arms programme.

Analysts have also said Japan has felt left out as the United States, China and South Korea take the lead in trying to confront Pyongyang's nuclear ambitions.

Tokyo's often testy relations with Beijing and Seoul have been further strained by feuds over disputed isles and recent remarks by Japanese politicians that appeared to try to justify its wartime aggression in Asia.

Mr Iijima, also an aide to former prime minister Jinichiro Koizumi when he held summits in Pyongyang, might succeed in reopening talks on the topic of Japanese citizens kidnapped by Pyongyang's agents decades ago, Keio's Mr Okonogi said.

Mr Abe told a parliamentary panel on Wednesday he was open to meeting North Korea's young leader, Kim Jong Un, if a summit would help resolve the issues of the abductees' fate and North Korea's nuclear and missile programmes.

Mr Kim Jong Un's father, the late Kim Jong-il, admitted during Mr Koizumi's 2002 visit that Pyongyang's agents had kidnapped 13 Japanese citizens in the 1970s and 1980s.

Five abductees were later repatriated. Pyongyang says the other eight are dead but Tokyo wants more information about them and others it believes were also kidnapped.