The US is beginning to detain imports of aluminum products suspected of being made through forced labor, particularly from China’s Xinjiang region, according to one of the world’s biggest shipping firms.

US Customs and Border Protection has begun issuing “detention notices” for such products, AP Moller-Maersk A/S said Tuesday on its website, adding that the action will most likely target aluminum used in automotive parts.

Allegations by the US and other countries of forced labor in Xinjiang, which Beijing has denied, are among a constellation of frictions between the world’s two biggest economies that increasingly see each other as their top strategic competitor. 

The CBP wasn’t immediately able to respond to a request for comment. 

Aluminum, and items made from the metal, now join cotton, tomatoes and polysilicon, in coming under scrutiny over links to the region, officially known as Xinjiang Uyghur Autonomous Region. The CBP in late December or early January began issuing detention notices naming aluminum as a fourth product, according to law firm Miller & Chevalier.

The enforcement falls under the Uyghur Forced Labor Prevention Act, which was signed by President Joe Biden in late 2021. Under the law, companies will have to document that goods sourced from or produced in Xinjiang aren’t sourced from forced labor.

“Companies importing aluminum products, or commodities with aluminum components, should be proactive in ensuring compliance with the UFLPA,” Maersk said in the advisory. “This may include conducting due diligence on their supply chain and implementing compliance programs.”

The impact on America’s aluminum markets will likely be small. The US imports virtually no primary aluminum from China, and less than 24,000 metric tons of value-added products per month, according to US trade data. 

Aluminum on the London Metal Exchange ended the day down 0.5%. Despite the lack of immediate market impact, physical traders said they’re seeking clarity on what the customs authorities are looking at.