"Arms negotiations between Russia and the DPRK are actively advancing," White House National Security spokesman John Kirby said, adding that a key focus of the talks was artillery ammunition for Moscow's forces.

Kirby noted that despite its denials, North Korea supplied infantry rockets and missiles to Russia last year for use by the privately controlled Wagner military group.

He said that recently Russian Defense Minister Sergei Shoigu had traveled to the Democratic People's Republic of Korea -- North Korea -- seeking to acquire additional munitions for the war.

"Since that visit, President (Vladimir) Putin and the leader of the DPRK, Kim Jong Un, have exchanged letters pledging to increase their bilateral cooperation," Kirby told reporters.

At the United Nations, the United States, United Kingdom, South Korea and Japan said in a joint statement that any such deal would violate Security Council resolutions forbidding arms deals with North Korea -- resolutions that Moscow itself had endorsed.

They said that following Shoigu's visit to Pyongyang, another group of Russian officials traveled to North Korea for follow-up talks on arms purchases.

"Russia is negotiating potential deals for significant quantities and multiple types of munitions from the DPRK to be used against Ukraine," they said.

"These potential deals could also include the provision of raw materials that would assist Russia's defense industrial base," they said.

"Any such arm deals would be a serious violation of resolutions the Security Council adopted unanimously after the DPRK past nuclear tests and ballistic missile launches," they said.