The United States has accused North Korea of supplying battlefield missiles and rockets to the Russian mercenary group Wagner for use in Ukraine.
The White House said the shipment violated UN Security Council resolutions and would announce further sanctions on Wagner.
Both North Korea and Wagner have denied the reports.
The UK government says the number of mercenary fighters in Ukraine has risen from 1,000 to 20,000.
The group has also recently been active in Syria and African countries and has been repeatedly accused of war crimes and human rights abuses.
“Wagner is seeking arms suppliers around the world to support his military operations in Ukraine,” White House national security spokesman John Kirby told reporters.
“We can confirm that North Korea has completed an initial shipment of weapons to Wagner, who paid for the equipment,” he said.
The White House says the mercenary group shipped infantry rockets and missiles from North Korea.
Mr Kirby said Wagner was spending more than $100m (£82m) a month in Ukraine.
He added that the group now rivals the Russian military in power.
British Foreign Secretary James Cleverly said that Britain agrees with Washington’s assessment.
“The fact that President (Vladimir) Putin is turning to North Korea for help is a sign of Russia’s desperation and isolation,” Mr. Cleverley said in a statement.
But Wagner’s owner Yevgeny Prigozhin denied the claim, calling it “gossip and speculation”, while North Korea’s foreign ministry dismissed the reports as “baseless”.
North Korea’s spokesman said its position was “not restored” and that arms deals between North Korea and Russia “never took place”.
