A US congressional commission on Tuesday called on American basketball stars to end endorsements of Chinese sportswear firms that use cotton grown in China’s Xinjiang region, warning against complicity in forced labor they say takes place there.

In a letter to the National Basketball Players Association, the chairs of the bipartisan Congressional-Executive Commission on China said more than a dozen NBA players had deals with the China-based Anta, Li-Ning and Peak sportswear firms prior to the publication of recent Western media articles saying the companies had backed continued use of Xinjiang cotton.

“Players have continued to sign new deals with Anta Sports,” the letter from Senator Jeff Merkley and Representative Jim McGovern added.

“We believe that commercial relationships with companies that source cotton in Xinjiang create reputational risks for NBA players and the NBA itself,” they said, noting that the US government had determined China was committing genocide and crimes against humanity in Xinjiang and barred cotton imports from the region.

“The NBA and NBA players should not even implicitly be endorsing such horrific human rights abuses,” the letter said.

It said reporting since 2018 had revealed authorities in Xinjiang had systematically forced minority Muslims to engage in forced labor and there was credible evidence forced labor existed in Xinjiang cotton production.

The letter, a text of which was provided to Reuters, said Anta, Li-Ning and Peak had publicly embraced Xinjiang cotton, “likely making them complicit in the use of forced labor.”

The NBPA did not immediately respond to a request for comment. China’s Washington embassy called allegations of forced labor “an outrageous lie.”

“The attempt by certain forces in the US and elsewhere to mess up Xinjiang and contain China will never succeed. The rock they are lifting will end up hitting their own toes,” it said in an email response to questions.

The NBA’s standing in China, its most important overseas market, deteriorated sharply after late 2019 when then-Houston Rockets general manager Daryl Morey expressed support for pro-democracy protests in Hong Kong and Beijing’s state television pulled NBA games off its channels.

The NBA said last July it was re-evaluating its training program in China following allegations of abuse of young players by local staff and harassment of foreign staffers in Xinjiang.

The NBA commissioner, Adam Silver, subsequently said in September the NBA’s long-standing engagement in China continued to have a “net positive” impact on the mutual understanding between the United States and the Communist nation.

By David Brunnstrom and Michael Martina; Editing by Jonathan Oatis and Karishma Singh