The US will boost sanctions on the Wagner group following North Korea's sale to it 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.