Seoul "will not stand by but respond firmly in collaboration with the international community," South Korea said.

U KRAINE’S PRESIDENT, Volodymyr Zelensky, declared last week that North Korea is sending troops to Russia, in effect joining the invasion as a co-combatant. The appearance of one of the world’s most erratic and heavily-armed nations in the fight might test the best resourced and well-rested defences.

It would be a "dangerous and highly concerning development" if North Korea was sending troops to help Russia in Ukraine, the United States said on Monday as South Korea and Britain warned of the high price Moscow would likely have to pay Pyongyang.