This week's satellite imagery showed a "dramatic" surge in rail traffic along the border between North Korea and Russia, a US monitor said Friday, amid concerns about possible arms transfers following last month's summit between the two countries.

Citing imagery captured on Thursday, Beyond Parallel, a project of the Washington-based Center for Strategic and International Studies, reported an "unprecedented" number of freight railcars totaling roughly 73 in number at Tumangang Rail Station in Rason, a North Korean border city with Russia.

The level of traffic is far greater than what the project has observed at the facility during the past five years, even compared to pre-pandemic levels, it said.

The development comes in the wake of a rare summit between North Korean leader Kim Jong-un and Russian President Vladimir Putin last month. The summit raised speculation that it might have led to a deal feared to help advance North Korea's weapons development and in return prop up Russia's war in Ukraine.

"Given that Kim and Putin discussed some military exchanges and cooperation at their recent summit, the dramatic increase in rail traffic likely indicates North Korea's supply of arms and munitions to Russia," the project said in a report.

"However, the extensive use of tarps to cover the shipping crates/containers and equipment makes it impossible to conclusively identify what is seen at the Tumangang Rail Facility," it added.

On Thursday, broadcaster CBS reported that the North began transferring artillery to Russia as Moscow continues its war in Ukraine.

Seoul and Washington have criticized any possible transfer of arms between Pyongyang and Moscow, warning they would flout multiple U.N. Security Council resolutions, which Russia itself voted for. (Yonhap)