South Korea warns it could reopen discussions regarding supply of arms to Ukraine should Pyongyang and Moscow continue building military ties.

South Korea has pledged it will take measures to counter military cooperation between Russia and North Korea despite denials from Pyongyang that it has sent troops to support Moscow’s war on Ukraine.

Officials in Seoul said on Tuesday that the alliance between Russia and North Korea represents an international threat and pledged that South Korea will work with its allies to implement countermeasures, potentially including the provision of arms to Ukraine.

South Korea summoned the Russian ambassador on Monday after it accused the North of sending 1,500 soldiers to fight alongside Moscow’s forces in Ukraine. North Korea has dismissed Seoul’s assertion as “groundless rumours”, but Seoul on Tuesday swatted that aside, promising action.

“Sending troops to Russia’s illegal war of aggression in Ukraine is a significant security threat not only to our country but also to the international community,” the National Security Council said.

“Following the deployment of North Korean combat troops, the government will implement phased countermeasures,” it added.

A senior official in President Yoon Suk-yeol’s office said diplomatic, economic and military measures are being prepared, adding that South Korea could revisit its earlier decision not to send arms to Kyiv.

Seoul has been under pressure from some Western countries and Kyiv to provide Ukraine with lethal weapons but has so far focused on nonlethal aid, including demining equipment.

“We would consider supplying weapons for defensive purposes as part of the step-by-step scenarios, and if it seems they are going too far, we might also consider offensive use,” the official told reporters.

North Korea in recent months has denied accusations from the South as well as the United States that it has been supplying weapons to Russia’s forces.

Late on Monday, it slammed the claim from South Korea’s spy agency that it has now sent special forces personnel to Russia’s Far East for training in preparation for deployment in Ukraine.

“As for the so-called military cooperation with Russia, my delegation does not feel any need for comment on such groundless, stereotyped rumours,” a North Korean representative said at a committee meeting during the UN General Assembly.

Seoul’s accusations, the official said, are “aimed at smearing the image of [North Korea] and undermining the legitimate, friendly and cooperative relations between two sovereign states”.

Russia has not confirmed Pyongyang’s troop deployment but defended its military cooperation with the North, saying on Monday that it “is not directed against the interests of South Korea’s security”.

Pyongyang and Moscow have been allies since North Korea’s founding, and Russia’s full-scale invasion of Ukraine in 2022 has drawn them closer.