South Korea's intelligence agency says North Korea has dispatched about 1,500 special forces troops to Vladivostok in Russia's Far East this month.

The officials said North Korea has begun participating in Russia's aggression against Ukraine.

The National Intelligence Service made the announcement on Friday.

The officials said the soldiers were transferred from three locations, including a point near the northeastern city of Chongjin, from October 8 to 13, using Russian Navy transportation ships.

They revealed satellite imagery showing what appears to be at least one Russian ship near Chongjin Port to transport soldiers.

They also said they believe the soldiers will be separated into groups to undergo training by the Russian military in Vladivostok, Khabarovsk and elsewhere, and that they will then be deployed to the battle front in Ukraine.

They claimed that the soldiers have been provided with Russian military uniforms and arms, and that fake certificates have been issued to disguise them as people from Russia's ethnic minorities.

The officials added that a second dispatch is set be carried out soon.

South Korean media quoted intelligence sources as saying the number of North Korean soldiers to be sent to Russia will reach around 12,000.