The White House has revealed that North Korea has provided Russia with ballistic missiles and missile launchers that were used in recent attacks on Ukraine.
National Security Council spokesman John Kirby said that this was a breach of UN sanctions against North Korea, and that the US and its allies would raise the matter at the UN Security Council.
He also said that Russia intended to purchase missiles from Iran, another ally of North Korea.
The US has been supporting Ukraine in its fight against Russian-backed separatists since 2014, when Russia annexed Crimea and sparked a war in eastern Ukraine.
The conflict has intensified in recent weeks, with Russia deploying tens of thousands of troops near the border with Ukraine, and launching aerial and artillery strikes on Ukrainian positions.