The US will boost sanctions on the Wagner group following North Korea's sale to the group of infantry rockets and missiles last month, in violation of UN Security Council resolutions, said White House national security spokesman John Kirby.

"Wagner is searching around the world for arms suppliers to support its military operations in Ukraine," Kirby told reporters.

"We can confirm that North Korea has completed an initial arms delivery to Wagner, which paid for that equipment," he said.

Kirby said the Wagner group, which is independent of the Russian defense establishment and is leading a bloody siege of Bakhmut, Ukraine, is spending more than $100 million each month in its Ukraine operations.

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

- 'Sign of desperation' -

In a statement, British Foreign Secretary James Cleverly said the UK concurs with the American assessment that North Korea delivered arms to Russia for the Wagner group in violation of UN resolutions.

"The fact that President Putin is turning to North Korea for help is a sign of Russia's desperation and isolation," Cleverly said.

"We will work with our partners to ensure that North Korea pays a high price for supporting Russia's illegal war in Ukraine."

- Close to Putin -

The Wagner group is controlled by Yevgeny Prigozhin, a businessman once called "Putin's chef" for his work catering dinners for Vladimir Putin before and after he became the Russian president.

Prigozhin -- who also controls the notorious St. Petersburg internet "troll farm" that allegedly disrupted the 2016 US election -- has been a vocal critic of the Russian defense establishment's handling of the war in Ukraine.

His mercenary-like army has been carrying out operations, ostensibly private but implicitly approved by the Kremlin, in Syria, Libya, Sudan, the Central African Republic and other countries in Africa.

In several locations they have been accused of participating in atrocities.

In Ukraine, the group has served as an elite special forces-type operation that has better training, equipment and supplies than the mainstream Russian military.

- Convict recruits sent to front -

But Wagner has taken significant casualties, and Prigozhin has relied on prisons to supply Wagner with convicts to fill out its ranks.

Kirby estimated that the Wagner force now numbers about 50,000, including 10,000 skilled "contractors" and 40,000 convicts.

In Bakhmut and other areas of heavy fighting, Ukraine forces say that the relatively untrained convicts have been forced to the front, where many have been killed or injured.

According to US information, Kirby said, 1,000 Wagner fighters have been killed in the fighting in recent weeks, 90 percent of them convicts.

"It seems as though Mr. Prigozhin is willing to just throw Russian bodies into the meat grinder in Bakhmut," he said.

Kirby said Prigozhin appeared more interested in "influence peddling at the Kremlin" than protecting his troops.

"For him, it's all about how good he looks to Mr. Putin, and how well he's regarded at the Kremlin," Kirby said.