Earlier, US and North Korean officials met on Wednesday for 30 minutes on the sidelines of six-nation talks in Beijing.
"There was a meeting between North Korea and the United States on the sidelines of the talks today," a Japanese official told reporters.
A South Korean official said the meeting took place in the same conference room as the six-way discussions, rather than a private venue.
"US-North Korean bilateral contact lasted 30 minutes. We saw it, we were in the same conference room. We could see it," a South Korean official said.
Pyongyang had initially insisted on discussing the simmering nuclear stand-off only with Washington, but agreed to the multilateral talks on condition that the two countries discussed the issue bilaterally as well.
South Korea also denied Russian reports that North Korea told participants that it had no nuclear weapons.
"There was no such expression indicating if it has nuclear weapons or not," Wi Sung-Lac, director general of North American affairs at the South Korean foreign ministry said when asked whether North Korea had made the comments.
Wi is the deputy chief delegate of the South Korean team at the talks and was in the conference venue for the duration of the discussions on Wednesday.
"US-North Korean bilateral contact lasted 30 minutes. We saw it, we were in the same conference room. We could see it"
South Korean official
Russia's ITAR-TASS news agency had quoted North Korea's chief negotiator, Kim Yong-Il, as saying at the start of the talks that Pyongyang "has no nuclear weapons... and has no goals of developing them".
At the same time, if Washington does not take Pyongyang's security concerns into account, then North Korea "will create even more powerful ways of containing" the threat from the United States, said the negotiator, according to ITAR-TASS.
The six-way talks on the North's nuclear weapons development opened on Wednesday and are scheduled to run until Friday, with China, North and South Korea, the United States, Japan and Russia participating.