Washington says North Korea is delivering arms to the Wagner Group, for Russia’s use in Ukraine.

The White House on Thursday warned that North Korea had delivered arms supplies to the Wagner Group, a Russian paramilitary force with close ties to President Vladimir V. Putin.

Yevgeny V. Prigozhin, founder of the Wagner Group, denied the assertion as “gossip and speculation,” according to Reuters.

John Kirby, a spokesman for the National Security Council, said Mr. Prigozhin is spending about $100 million a month to deploy Wagner mercenaries in Ukraine. The paramilitary group has played a central role in Russia’s efforts to storm the city of Bakhmut in Donetsk Province, which borders Luhansk.

North Korea last month delivered infantry rockets and missiles to Russia for Wagner’s use, Mr. Kirby said.

Mr. Kirby said the deliveries by North Korea “will not change battlefield dynamics in Ukraine, but we are certainly concerned that North Korea is planning to deliver more military equipment.”

He said the Biden administration planned to report the deliveries as a violation of U.N. Security Council resolutions.

The comments on Thursday came a day after the Commerce Department tightened sanctions on the security company. The updated restrictions are designed to make it more difficult for the Wagner Group to obtain supplies on the open market.

American officials believe that tightening export controls and other sanctions on the Wagner Group will force it to look for weapons, ammunition and other equipment from pariah states like North Korea and Iran.

Wagner has about 50,000 people fighting in Ukraine, including 10,000 contractors and about 40,000 convicts that Mr. Kirby said the company has recruited from Russian prisons. The company has also sent mercenaries to support the Kremlin’s military campaigns in Africa and the Middle East.

The United Kingdom agreed with the United States’ conclusion that North Korea was helping to arm Wagner fighters in Ukraine.

“The fact that President Putin is turning to North Korea for help is a sign of Russia’s desperation and isolation,” the British foreign secretary, James 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.”