In response to Moscow’s embrace of Pyongyang, Chang Ho-jin, the South Korean national security advisor, told reporters on Thursday that Seoul will review the possibility of supplying weapons to Ukraine. Currently, it has provided only non-lethal aid such as mine clearance equipment.

South Korea’s presidential office said: “Any cooperation that directly or indirectly helps North Korea increase its military power is a violation of UN Security Council resolutions and is subject to monitoring and sanctions by the international community.”

Russia’s agreement with North Korea “should be a concern to any country that cares about maintaining peace and stability,” said John Kirby, the spokesman for the US national security council.

Moscow’s agreement with Pyongyang revives an earlier Cold War-era defence pact. According to North Korean state media, if either country experiences an “armed invasion”, then the other “shall provide military and other assistance with all means in its possession without delay”.

North Korea’s leadership also signalled it may take action after a balloon launch by anti-Pyongyang activists on Thursday night, floating thousands of propaganda leaflets, US dollar bills, and USB sticks containing K-pop songs and South Korean TV dramas across the border from the South.