The six-nation talks over the next three days are expected to focus on how to verify the North's accounting of its program, but negotiators have said they expect the process to be difficult.
U.S., South Korean and Japanese nuclear negotiators met Sunday in Beijing for discussions ahead of the talks, which start Monday.
The officials discussed U.S. nuclear negotiator /*Christopher Hill*/'s meetings on Thursday and Friday with his North Korean counterpart in Singapore, Japan's /*Akitaka Saiki*/ told reporters.
"After hearing about what was discussed between the U.S. and North Korea, it appears a big gap still remains," Saiki said.
"Regarding how to narrow the gap, it's up to each party's efforts from tomorrow. I think negotiations are going to be tough."
South Korea's top nuclear negotiator, Kim Sook, who also met Sunday with his Russian counterpart, Alexei Borodavkin, said they sought to develop specific verification methods.
"The verification protocol should be more specific and clear in guaranteeing the implementation of the verification," Kim Sook quoted Borodavkin as saying.
Kim earlier also expressed pessimism about the talks.
"I am not optimistic at all," Kim said, according to South Korea's Yonhap news agency. Referring to Hill's meeting with the North Korean envoy in Singapore, Kim said "there was no explicit outcome or new compromise." He did not provide details.
North Korea vowed Saturday to ignore Japan at the talks, citing Tokyo's refusal to send aid to the impoverished country as part of the disarmament agreement. North Korea has issued similar warnings in the past, but Tokyo has continued to attend the negotiations that began in 2003.
Under the agreement, North Korea pledged to disable its nuclear reactor in exchange for 1 million tons of heavy fuel oil or equivalent aid, half of which has been delivered. Japan has refused to join the four other countries - China, Russia, South Korea and the United States - in providing the aid until North Korea addresses the kidnapping of more than a dozen Japanese in the 1970s and '80s.