Bogatyr Komir produces 41 million tonnes of coal a year, which it sells in Kazakhstan and Russia, mostly to power plants.
U.S. sanctions could hamper the venture's equipment upgrade project, Samruk, a unit of sovereign fund Samruk Kazyna, said in a written reply to questions from Reuters.
"Probable sanctions-related risks have been identified, including with regards to Bogatyr Komir, in which Rusal indirectly owns a 50 percent stake," the firm said.
Samruk said it had contacted ThyssenKrupp, a supplier of equipment for the venture, and the Eurasian Development Bank, asking them to confirm whether they would continue working with Bogatyr Komir.
It provided no other details.