Pyongyang has supplied up to 1,000 containers of "equipment and munitions" in "recent weeks", National Security Council Spokesperson John Kirby said.
Officials also released photos of what they said were 300 containers assembled for transport in Najin, North Korea.
Last month, North Korean leader Kim Jong Un visited Russia to discuss potential military cooperation.
Moscow's military is believed to be burning through huge amounts of artillery shells and missiles in its ongoing invasion of Ukraine, and has been seeking to replenish its supplies from some of its isolated allies.
Some analysts believe that Mr Kim's regime could be sitting on huge stores of arms, but could be reluctant to hand over too much given its relative lack of resources.
US intelligence agencies tracked the deliveries, which officials said took place between 7 September and 1 October.
Speaking at a news conference on Friday, Mr Kirby said the equipment was exported via sea and rail to a supply depot in southwestern Russia, near Tikhoretsk, about 180 miles (290km) from the Ukrainian border.
Mr Kirby did not specify the nature of the munitions he says were supplied by Mr Kim's regime, but the US has previously accused Moscow of purchasing rockets and artillery shells from Pyongyang.
