Lambert and Jo just had a brief five-minute talk during a reception for the participants on Thursday.

Lee Do-hoon, South Korea's chief nuclear negotiator, was also present at the conference but did not hold talks with Jo other than an exchange of greetings.

The potential encounter between Lambert and Jo had drawn much attention as nuclear negotiations between Washington and Pyongyang remain in a stalemate after their last working-level talks in Sweden in early October ended without a breakthrough.

The nuclear nonproliferation conference, hosted by Russia's Center for Energy and Security Studies every two to three years, has served as a chance for the United States and North Korea to sound out each other's positions at times of high tension.

According to participants at the event, Jo appeared to be trying to shun contacts with the US or South Korean officials.

During a session at the conference on Friday, Jo urged the US to make a forward-looking decision by the end of the year, saying that the window for dialogue is closing bit by bit every day.

Lambert and Lee were present at the session but did not show any immediate reaction to Jo's remarks.

The US and South Korean envoys left Moscow on Saturday to return home. Jo was to leave Moscow the next day to return home via China. (Yonhap)