US officials have accused North Korea of supplying vast amounts of military hardware to Russia for use in Ukraine.
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.
Since Russia launched its invasion of Ukraine in February 2022, US officials have consistently voiced concerns that North Korea has supplied munitions to Russia.
"We condemn [North Korea] for providing Russia with this military equipment, which will be used to attack Ukrainian cities, kill Ukrainian civilians and further Russia's illegitimate war," Mr Kirby told reporters.
He said the deliveries by North Korea violate UN Security Council resolutions "which is why we will continue to aggressively raise these arms deals at the UN alongside with our allies and partners".
In July, Russian Defence Minister Sergei Shoigu visited the country with a military delegation and met with Mr Kim, who displayed a number of weapons systems - including the Hwasong intercontinental ballistic missile (ICBM).
And in September, Mr Kim met with Russian President Vladimir Putin at the Vostochny space centre in Russia's far east.
Observers say that North Korean weapons would only give a short-term boost to Russia's war effort. They point to how Moscow, with hugely depleted ammunition, is relying on older, more unreliable artillery shell stocks.
And speaking recently at a ceremony to mark his retirement as chairman of the US Joint Chiefs of Staff, Gen Mark Milley said he was "sceptical" that any such deliveries would play a decisive role in the conflict.
But it comes as the US has been forced to pause plans to send an additional $6bn in military aid to Kyiv, amid an ongoing budget row in the House of Representatives.
President Biden said earlier this week that the temporary agreement between House Democrats and Republicans may force him to find alternative ways to fund Ukraine's war effort.
