The United States will impose additional sanctions next week against Russian private military company the Wagner Group, which US officials say has been helping Russia's military in the Ukraine war, the White House said on Friday.

White House national security spokesperson John Kirby said the US Treasury Department will designate Wagner as a significant Transnational Criminal Organisation. He said Wagner's growing clout in Russia was causing tensions among the country's military leadership.

"In coordination with this designation, we will also impose additional sanctions next week against Wagner and its support network across multiple continents. These actions recognise the transcontinental threat that Wagner poses, including through its ongoing pattern of serious criminal activity," Kirby told reporters at the daily White House briefing.

Declaring Wagner a Transnational Criminal Organisation under US executive order 13581 freezes any US assets of Wagner and prohibits Americans from providing funds, goods, or services to the group.

"With these actions, and more to come, our message to any company that is considering providing support to Wagner is this: Wagner is a criminal organisation that is committing widespread atrocities and human rights abuses, and we will work relentlessly to identify, disrupt, expose, and target those assisting Wagner," Kirby said.

Last month the White House said the Wagner Group had taken delivery of an arms shipment from North Korea to help bolster Russian forces in Ukraine, in a sign of the group's expanding role in that conflict.

North Korea's foreign ministry had called the report groundless and denounced the United States for providing lethal weapons to Ukraine. Wagner owner Yevgeny Prigozhin last month denied taking delivery of arms from North Korea and characterised the report as "gossip and speculation."

Prigozhin, asked for reaction to Kirby's Friday comments, did not directly address the US accusations, but said they showed that Wagner and the Americans were "colleagues" who were both involved in "dismantling criminal clans," an apparent reference to Russia's claim that its war is designed to remove what it calls Ukraine's illegitimate government.

The White House on Friday made public an image dated November 18 that it said showed Russian rail cars travelling between Russia and North Korea.

Kirby said North Korea delivered infantry rockets and missiles into Russia for use by Wagner and that the imagery showed five Russian railcars that travelled from Russia to North Korea. "On November 19, North Korea loaded the railcars with shipping containers, and the train returned to Russia," he said.

"While we assess that the amount of material delivered to Wagner has not changed battlefield dynamics in Ukraine, we expect that it will continue to receive North Korean weapons systems," he said.

Kirby said Russian President Vladimir Putin has been increasingly turning to Wagner for military support, causing some tensions in Moscow.

"We are seeing indications, including in intelligence, that tensions between Wagner and the Russian Defence Ministry are increasing," he said. "Wagner is becoming a rival power centre to the Russian military and other Russian ministries."

He said the United States assesses that Wagner currently has about 50,000 personnel deployed to Ukraine, including 10,000 contractors and 40,000 convicts recruited from Russian prisons.

"Our information indicates the Russian Defence Ministry has reservations about Wagner's recruitment methods. Despite this, we assess that it is likely that Wagner will continue to recruit Russian prisoners," he said. (Reuters)