At a training site in Russia, the five North Korean soldiers had grins on their faces and guns in their hands. Then they began singing a cappella, belting out a Soviet-era song called “Katyusha" about a young woman bidding farewell to soldiers—in translated Korean-language lyrics.

North Korean leader Kim Jong Un made a rare visit to Russia's embassy in Pyongyang with his daughter on Friday and reaffirmed "the invincible alliance" between the two countries, while Russia's leader greeted North Korean generals in Moscow.