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.

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.

A visit by Kim later this month to Russia's far-eastern port city of Vladivostok is partly to discuss supplying Moscow with weapons for the war in Ukraine, the New York Times reported this week.

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.

An annual summit between the three has not taken place since 2019 because of tensions, largely over Japan's wartime past.

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.

North Korea, which maintained strict border restrictions during the COVID-19 pandemic, has been reopening slowly in recent weeks, with increased train crossings, and the resumption of flights, with China.

Before the pandemic, China hosted the largest number of North Korean workers abroad, with as many as half of the estimated 100,000 people earning more than $500 million a year.