Washington is concerned about potential weapons deals from Pyongyang to supply the Kremlin's struggling war in Ukraine, including significant quantities and multiple types of munitions, the White House said.
Following Defense Minister Sergei Shoigu's recent trip to Pyongyang for discussions on these potential arms deals, Russian President Vladimir Putin and North Korean leader Kim Jong Un exchanged letters pledging to increase their cooperation, National Security Council spokesman John Kirby told reporters.
These letters "were more at the surface level," Kirby said, but he added that these high level discussions may continue in the coming months.
The White House said a particular focus was artillery munitions, as Russia's military fights to hold on to occupied land in the face of Ukraine's counteroffensive.
"It's a gunfight and both sides are blazing away with artillery," Kirby said.
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"We urge the DPRK to cease its arms negotiations with Russia and abide by the public commitments that Pyongyang has made to not provide or sell arms production," Kirby continued, using the acronym for the Democratic People's Republic of Korea.
He added a warning to all others associated with the deals. "And of course, we'll take action directly by exposing and sanctioning individuals and entities working to facilitate arms deals between these two countries."
Any arms deal between North Korea and Russia would directly violate a number of United Nations Security Council resolutions.
The U.S. has previously accused North Korea of supplying Russia with munitions to support its war in Ukraine, with Pyongyang receiving food and energy from Moscow in return. The Treasury Department announced new sanctions earlier this month targeting three entities tied to a network trying to support arms deals between the two countries and avoid U.S. sanctions.
Russia and North Korea have both denied any deal to transfer arms, but the two countries have had a close relationship dating to the Cold War and have made no secret of their growing military collaboration in the face of Western sanctions and scorn.
"Moscow and Pyongyang maintain good, mutually respectful relations, we mean to develop [them] further," Kremlin spokesman Dmitry Peskov said in a daily briefing call Friday. "Contacts are being made at various levels. This is our neighbor, a very important neighbor in this region, so these relations will continue to develop," he added.
Also Thursday, North Korea conducted missile launches that it said were simulated "scorched earth" nuclear strikes on South Korea as it protests against the South's joint military exercises with the U.S.