On Friday, Jo Chol-su, director-general of the North American department at North Korea's foreign ministry, urged the US to make a forward-looking decision by the end of the year.

"We have given considerably much time to the US, and we will wait for some results until the end of this year," Jo said in an answer to a participant's question during a session of the 2019 Moscow Nonproliferation Conference.

"Though we expect everything to go into a positive direction, I want to say that the window for opportunity is closing bit by bit every day," he added.

The three-day conference brought together 300 experts and officials from around 40 countries to discuss nuclear nonproliferation and arms control, according to according to the organizer, Russia's Center for Energy and Security Studies.

Lee and Lambert merely exchanged brief greetings with Jo on Friday. At a forum reception on Thursday, Lambert and Jo had a five-minute talk. No details were immediately available.

Also Friday, Lee held talks with Russian Deputy Foreign Minister Igor Morgulov and the two shared the view that North Korea and the US need to maintain the dialogue momentum, South Korea's foreign ministry said.

Morgulov reportedly had a separate meeting with Jo, though no details were immediately available.

Lambert also met with Morgulov and Russian Ambassador-at-Large Oleg Burmistrov in Moscow on Friday to discuss continued US-Russia cooperation on North Korea.

"The conversation was candid and constructive, and Special Envoy Lambert emphasized the importance of continued close coordination with Russia on North Korea," the US Embassy in Moscow said in a statement posted on its website.

On Friday, Lee Moon-hee, director-general for North Korean nuclear affairs at the South Korean foreign ministry, held two separate meetings with Burmistrov and Masami Tamura, deputy director-general of Asian and Oceanian Affairs Bureau at Japan's Foreign Ministry. (Yonhap)