White House national security spokesperson John Kirby said on Friday that Wagner, which has been supporting Russian forces in their invasion of Ukraine and claiming credit for battlefield advances, would be designated a significant Transnational Criminal Organization.
A letter in English addressed to Kirby and posted on the Telegram channel of Wagner founder Yevgeny Prigozhin's press service read: "Dear Mr Kirby, Could you please clarify what crime was committed by PMC Wagner?"
Kirby called Wagner "a criminal organization that is committing widespread atrocities and human rights abuses".
Last month, the White House said Wagner had taken delivery of an arms shipment from North Korea to help bolster Russian forces in Ukraine.
North Korea's Foreign Ministry called the report groundless and Prigozhin at the time denied taking such a delivery, calling the report "gossip and speculation".
Washington had already imposed curbs on trade with Wagner in 2017 and again in December in an attempt to restrict its access to weaponry.
The European Union imposed its own sanctions in December 2021 on Wagner, which has been active in Syria, Libya, the Central African Republic, Sudan, Mozambique and Mali, as well as Ukraine.
Prigozhin has described Wagner as a fully independent force with its own aircraft, tanks, rockets and artillery.
He is wanted in the United States for interference in US elections, something that he said in November he had done and would continue to do.