BEIJING—U. S. trade officials issued another warning to Chinese e-commerce giant Alibaba Group Holding Ltd., saying brand owners continue to complain that its online sales platforms are still major sellers of counterfeit goods.

In a report on piracy in global markets released on Thursday, the U.S. Trade Representative said Alibaba should intensify its efforts to work with companies to eliminate fakes on platforms like Taobao, the company’s massive online bazaar.

“Brand...