John Kirby made the statement at a media briefing on Thursday, the Reuters news agency reported.
The White House official said that according to US intelligence, North Korea "recently provided Russia with ballistic missile launchers and several ballistic missiles."
He added that Russia had used "multiple" North Korean missiles in its recent air strikes on Ukraine.
Kirby told reporters that North Korea's transfer of arms to Russia was a "concerning escalation," and that the United States would raise the issue with the United Nations Security Council.
Washington will also impose additional sanctions against those facilitating the weapons deals, the US official announced.
Russia is additionally making efforts to buy ballistic missiles from Iran and could acquire them by spring 2024, the Institute for the Study of War reported, citing sources in the American administration.
Russia may be "intensifying efforts to source ballistic missiles from abroad" because these missiles "appear to be more effective at striking targets in Ukraine in some circumstances," the US think tank added in its latest report, issued on Thursday night.
Kyiv said Moscow had fired at least 500 attack drones and missiles against Ukraine between December 29 and January 2, aiming to cause maximum destruction.
On December 29, Russia launched the largest attack of the war, killing at least 39 Ukrainian civilians, according to news outlets.
Friday is day 681 of Russia’s war on Ukraine.
Russia invaded Ukraine on February 24, 2022, launching the largest military campaign in Europe since World War II.
Source: Reuters, ISW
