South Korean TV broadcast footage in October appearing to show North Korean troops in a Russian military base.Kim Jae-Hwan/SOPA Images/LightRocket via Getty ImagesSouth Korea says North Korean troops are being trained by Russia to fight in Ukraine.The Kremlin said the cooperation between the two was "within the framework of international law."Russia is worried that South Korea will send weapons to help Ukraine, according to a US think tank.The Kremlin's alliance with North Korea may have bolstered its campaign in Ukraine, but it's come at a potentially serious cost.South Korea's government this week signaled its fury over the growing alliance between the two countries.In a meeting with Russia ambassador Georgiy Zinoviev, South Korea demanded the withdrawal of North Korean soldiers which it said were being trained by Russia to fight in Ukraine.Yoon Suk Yeol, a senior official at the office of South Korea's president, said on Tuesday that South Korea is now mulling sending weapons to help Ukrainian troops.In response, the Kremlin seemed eager to reassure South Korea.