The statement comes a month after North Korean leader Kim Jong Un traveled to Russia's far east and met President Vladimir Putin, sparking concern among Kyiv's allies over the possibility of a potential arms deal.

"We condemn the DPRK for providing Russia with this military equipment, which will be used to attack Ukrainian cities, kill Ukrainian civilians, and further Russia's illegitimate war," White House national security spokesman John Kirby told reporters, using North Korea's official name.

"We will continue to monitor for any additional DPRK arms shipments to Russia."

In exchange for its support, Kirby said, "Pyongyang is seeking military assistance from Russia, including fighter aircraft, surface-to-air missiles, armored vehicles, ballistic missile production equipment or other materials and other advanced technologies."

"This expanding military partnership between the DPRK and Russia including any technology transfers from Russia to the DPRK undermines regional stability and the global non-proliferation regime," he added.

According to a graphic provided by the White House, between September 1 and October 1, a load of containers was shipped by sea from Najin, North Korea, to Dunay, Russia.

They were then delivered by rail to an ammunition depot near Tikhoretsk in southwestern Russia, some 290 kilometers (180 miles) from the Ukrainian border.

North Korea, which the US has previously accused of supplying shells to Russia's Wagner Group, is a mass producer of conventional weaponry and is known to be sitting on large stocks of Soviet-era war material -- albeit in unknown condition.

Russia has ramped up production of shells this year to a forecast 2.5 million, but analysts have suggested that could fall short of its needs on the battlefield.

Moscow's forces are firing about 60,000 rounds per day, according to Ukrainian figures.

While Russia said no agreements were signed during Kim's visit, Putin said he saw "possibilities" for military cooperation.

Russia and North Korea, historic allies, are both under a raft of global sanctions -- Moscow for its invasion of Ukraine, and Pyongyang for its testing of nuclear weapons.

Kirby 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."