A US think tank says satellite images show that North Korea may have begun transferring weapons to Russia.

The Center for Strategic and International Studies on Friday released findings from the analysis of satellite imagery of North Korea's rail facility at the border with Russia. The images were captured the previous day.

CSIS researchers say they have confirmed approximately 73 freight railcars at the facility. They say it's an unprecedented number.

North Korean leader Kim Jong Un and Russian President Vladimir Putin held talks in September. There was speculation that Putin would ask Kim to supply munitions.

The researchers added that five days after the summit, there was a "gradual but steady increase" in the number of freight cars at the North Korean facility. However, the contents cannot be confirmed as the cars are covered with tarpaulins.

The US government is on the alert for any military transfers between the two countries that would violate multiple UN Security Council resolutions and be subject to additional sanctions.