South Korean President Yoon Suk Yeol said on Wednesday any attempt to cooperate with North Korea on military affairs in a way that damages international peace must immediately halt, APA reports citing Reuters.
Yoon made the comment at a summit meeting with Southeast Asia's ASEAN bloc countries in Jakarta, Indonesia, his office said.
The office did not elaborate but his comment comes amid reports that arms negotiations between North Korea and Russia are actively advancing and that the North's leader, Kim Jong Un, plans to visit Russia soon to meet President Vladimir Putin.
"Attempt at military cooperation with North Korea that harms international peace must immediately halt," Yoon's office quoted him as saying at a meeting with the leaders of ASEAN countries.
North Korea and Russia have denied they were in arms negotiations.
Russia's defence minister, Sergei Shoigu, said earlier the two countries planned to conduct joint military exercises.
At a later meeting with ASEAN leaders, together with Japanese Prime Minister Fumio Kishida and Chinese Premier Li Qiang, Yoon said South Korea would work closely with the two Asian neighbours with the goal of resuming their three-way talks on improving ties.
Yoon called for vigilance to help stop illicit North Korean activities that fund its nuclear and missile programmes, including sending workers abroad to earn foreign currency.
Under a U.N. Security Council resolution adopted in 2017 and backed by China, U.N. member countries are required to send back all North Korean workers and not take them on again.