The analysts suggested the build-up could be evidence of weapons being transferred from North Korea to Russia to be used to prop up its invasion of Ukraine.

“Satellite imagery of North Korea’s Tumangang Rail Facility at the border with Russia captured on October 5, 2023, shows an unprecedented number of freight railcars, totaling approximately 73 railcars,” the think tank wrote in a summary of its findings.

“The level of rail traffic is far greater than what Beyond Parallel has observed at the facility during the past five years, even compared to pre-COVID-19 levels.”

Excessive tarps and other coverings make it impossible to determine what exactly the railcars in the photos are carrying, they said.

American officials have not commented publicly on the report, which came as CBS News, citing unnamed U.S. officials, reported Friday that North Korea has indeed started transferring weapons to Russia. The broadcaster did not give further details or specify what type of weapons.

North Korea and Russia share a 10.7-mile land border as well as a maritime border measuring 12 nautical miles.

North Korean leader Kim Jong Un last month traveled by rail on an armored train to Russia for a summit with Russian President Vladimir Putin. The two toured weapons factories and other facilities during the state visit, where Kim invited Putin to take a reciprocal trip to North Korea.

The visit was Kim’s longest international trip since taking power from his late father in 2011.

Putin later told Russian state media the meeting between the two leaders “opened a new chapter” in the relationship between the countries.

Kim is reportedly seeking advanced technology for his country’s space and missile programs. It wasn’t clear what any agreement between Russia and North entailed as far as payment.