White House national security spokesman John Kirby said Wagner, controlled by Yevgeny Prigozhin, a businessman close to President Vladimir Putin, has about 50,000 fighters in Ukraine, 80 percent of them drawn from prisons.

Kirby showed US intelligence photographs of North Korea supplying arms to Wagner for its Ukraine operations, and said the private army has become a rival to the formal Russian military.

The photographs, from November 18-19, show Russian rail cars entering North Korea, picking up a load of infantry rockets and missiles, and returning to Russia, he said.

He said the US Treasury was formally designating Wagner as a transnational criminal organization, putting it in league with Italian mafia groups and Japanese and Russian organized crime.

The designation will allow the wider application of sanctions on the group's sprawling global network, which includes mercenary operations as well as businesses in Africa and elsewhere.

Wagner "is a criminal organization that is committing widespread atrocities and human rights abuses," Kirby said.

"We will work relentlessly to identify, disrupt, expose and target those who are assisting Wagner," he said.

Kirby also said the United States had presented its intelligence on Wagner's North Korean purchase to the UN Security Council's unit on North Korea sanctions.

The arms transfers from North Korea are in direct violation of United Nations Security Council resolutions, Kirby said.

Kirby said there is evidence that Prigozhin's confidence in Wagner fighters' relative success in Ukraine has generated tensions in the Kremlin.

"Wagner is becoming a rival power center to the Russian military and other Russian ministries," Kirby said.

"Prigozhin is trying to advance his own interest in Ukraine and Wagner is making military decisions based largely on what they will generate for Prigozhin, in terms of positive publicity."