North Korea promised in February to shut down its Yongbyon reactor as the initial step of a six-nation aid-for-disarmament deal. But it has refused to comply due to a long-running row over frozen assets.

"Our top priority is to nail down whether North Korea is completely implementing the initial step," chief government spokesman Yasuhisa Shiozaki told a news conference.

"Without securing this, we will hold neither six-way talks nor a ministerial meeting of the six nations," Shiozaki said.

US chief negotiator Christopher Hill said on a visit to Japan Saturday that he hoped for six-nation negotiations on disarmament and a first meeting of the countries' foreign ministers both in July.

Hill last week paid a rare visit to the communist state, which said Monday that the row over the frozen assets had finally been settled.

UN inspectors were due back in North Korea on Tuesday for the shutting down of Yongbyon, the source of weapons-making plutonium.

Japan has taken the hardest line in the six-party disarmament talks, which also involve China, the two Koreas, Russia and the United States.

Japan has refused to fund February's deal due to the emotionally charged row over Pyongyang's past abductions of Japanese civilians in the 1970s and 1980s.

Prime Minister Shinzo Abe, who has campaigned on the abduction row throughout his career, reiterated that Japan would not establish diplomatic relations with North Korea.

"Without solving this issue, we cannot normalise relations. There is no change in this policy at all," Abe told reporters.

Japan has expressed unease about Hill's visit to North Korea, which has long sought to negotiate directly with the United States instead of six-way talks.

Russia's former prime minister Yevgeny Primakov, who is visiting Japan this week, also suggested he was unhappy with Hill's visit.

Members of the six-party talks "can take independent actions, but I doubt whether it is appropriate to do so without consultation with other parties," Primakov was quoted by Kyodo News as saying.

"It is only by making efforts to act on a common stance that we will be able to eventually denuclearise North Korea," he was quoted as saying.

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