The Treasury Department said it was freezing any U.S. assets and criminalizing transactions with Ali Yagoub Gibril, Central Darfur commander of the Rapid Support Forces, and an RSF major general involved in operational planning, Osman Mohamed Hamid Mohamed.
"The RSF military operation to encircle and besiege el-Fasher, North Darfur, has endangered the lives of hundreds of thousands of civilians," State Department spokesman Matthew Miller said in a statement.
"We stand ready to take additional measures against those individuals and institutions that actively escalate the war — including any offensive actions on el-Fasher," he said.
U.N. Secretary-General Antonio Guterres on Monday also voiced alarm over reports of heavy fighting in densely populated areas as the RSF seeks control of el-Fasher, the last major city in Darfur not under its control.
Tens of thousands of people have died, and millions have been displaced since war broke out in April 2023 between Sudan's army and the RSF after their head generals refused a plan to integrate.
The United States has led diplomatic efforts to stop the fighting but has seen limited success and leverage, with RSF commanders unlikely to hold major assets in the West affected by sanctions.
The RSF and Sudan's armed forces are seen as both wanting to secure a battleground victory, and each side has received support from outside players.
The U.S. special envoy for Sudan, Tom Perriello, is again traveling around the Middle East and Africa this week in hopes of making progress.
The United States has accused both sides of war crimes and charged that the RSF has carried out ethnic cleansing and crimes against humanity against the indigenous African-origin people of Darfur.
The RSF's predecessor, the Janjaweed militia, carried out a scorched-earth campaign in the arid western region that the United States at the time described as genocide.