If true, it would test the world's patience and throw the talks process into doubt.
Japan and host China, partners in the deadlocked negotiations along with the United States, Russia and the two Koreas, said on Friday that the talks were on for this week, but no exact date had been fixed.
China's top negotiator, Wu Dawei, flew to Pyongyang on Saturday and was expected to stay until Tuesday.
The regional powers hope to persuade impoverished North Korea to dismantle its nuclear weapons programmes in exchange for security guarantees and economic assistance.
"The North Korean foreign minister told me what he had in mind, what had caused North Korea not to be able to participate in the six-party talks scheduled for Monday," visiting Thai Foreign Minister Kantathi Suphamongkhon told reporters.
There has been no firm date for
resumption of the talks
"The North Koreans said that they are willing to dismantle their nuclear weapons as long as there is trust among the parties concerned.
"They say they are ready to dismantle and go back to the NPT (Non-Proliferation Treaty), allowing the International Atomic Energy Agency to step in, as long as there is trust among parties.
"I hope that the talks can resume at least by mid-September or within September at the latest," he added, without specifying what gave him that hope.
North Korea threw out IAEA inspectors at the end of 2002 and withdrew from the NPT in January 2003.
Kantathi met North Korean Foreign Minister Paek Nam-sun for about 90 minutes in Pyongyang on Saturday, followed by dinner.
The status of the six-party talks had been up in the air, with silence from all sides on a firm date to resume, after the participants agreed to a three-week recess in the last round which ended this month.
"We have given them a chance, maybe a last chance so to speak, to resolve the crisis in the region the way we all have been hoping for. Our patience would wear thinner and thinner"
Japanese government source
Japan said on Sunday no decision had been made, as far as it knew.
"As far as Japan is concerned, the date of the talks is still under discussion," Foreign Ministry spokesman Akira Chiba said in Tokyo.
Previous rounds of six-party talks have ended with simply an agreement to meet again.
Wu Dawei (R) has flown to
Pyongyang for talks
"If North Korea actually refused to return to the six-party forum this week, that would mean they would break the promise they had made to all other parties concerned," a Japanese government source said.
"We have given them a chance, maybe a last chance so to speak, to resolve the crisis in the region the way we all have been hoping for.
"Our patience would wear thinner and thinner. I believe particularly those in the US government would feel so disappointed and frustrated and their patience would wear very thin," the source added.
North Korea said on Saturday that Washington's decision to appoint a special envoy to monitor human rights in the country had cast a shadow over the talks.
Pyongyang, which has routinely accused the US of hostility in the talks and lack of trust, has been playing the nuclear card to win diplomatic and economic benefits since the standoff began in October 2002.
Washington said then that Pyongyang had admitted to a secret programme to enrich uranium, in violation of a 1994 agreement, a claim North Korea later denied.
Described by US President George Bush as part of an "axis of evil" along with Iran and pre-war Iraq, North Korea said for the first time this year it had nuclear weapons, arguing it needed them to deter a hostile United States.