The US said any arms deal would violate UN Security Council resolutions.
Russia is sending a delegation to North Korea to offer food in exchange for weapons, US national security spokesperson John Kirby has said.
Mr Kirby said any arms deal between North Korea and Russia would violate UN Security Council resolutions.
The US has previously accused North Korea of supplying arms to the Russian military and the Wagner group of Russia mercenaries.
Pyongyang has previously denied the claims.
Mr Kirby said the US is monitoring the situation, and the alleged deal, closely.
Earlier on Thursday, the US Treasury blacklisted a Slovakian man for acting as a broker between Russia and North Korea.
The Treasury said Ashot Mkrtychev, 56, had arranged sales and helped organise deals that would enable North Korea to ship weapons to Russia in late 2022 and early 2023.
In return Pyongyang received cash, commercial aircraft, commodities and raw materials, they said.
Featuring on the sanctions blacklist bans American businesses from having dealings with Mr Mkrtychev and freezes his US assets.