Chinese authorities have released on bail three activists who had been detained after investigating labor conditions at a factory that produced shoes for Ivanka Trump and other brands.

The three activists walked out of a police station in Ganzhou, a city in southeastern Jiangxi province, on Wednesday, the final day of their legally mandated 30-day detention period limit.

Su Heng, Li Zhao and Hua Haifeng, who were working on behalf of New York City-based China Labor Watch (CLW), were investigating two plants owned by footwear producer Huajian Group when they were detained by police.

China said that they were being investigated on suspicion of using "spying and other monitoring equipment" while a Chinese media report cited police as saying the men had confessed to using recording devices hidden inside watches to steal "business secrets" from the factories.

The United States had urged Chinese authorities to release the men immediately.

"It is very rare to see defenders of the rights of workers released on bail," Patrick Poon, China researcher at Amnesty International, told AFP.

"It might be due to international pressure and the nature of the case that they are released on bail," he said, adding, "we also hope that they will be given a fair trial".

One of the activists, Hua Haifeng, carried his 3-year-old son in his arms as he walked out with his wife and other family members.

"I will speak to everyone in a few days' time after we organize. I'm happy to be out. I just want to spend some time with my family," Hua told The Associated Press. "I appreciate the media following my case the last month but I'm not ready to speak yet."

Hua declined further comment but said he had not been mistreated. People released in politically sensitive cases tend to have conditions attached to their release that restrict them from speaking to the media.

The two factories under investigation by the activists were in the city of Dongguan in the southern province of Guangdong, and Ganzhou in neighbouring Jiangxi province.

They discovered that employees were forced to work overtime while being paid less than the minimum wage, according to CLW director Li, who has alleged that the factory issued fake pay slips that showed wages greater than the workers' actual compensation.

Aside from Ivanka Trump, Huajian also manufactures products for Coach, Nine West, Karl Lagerfeld, and Kendall + Kylie, Li said.

A spokesman for the company told AFP in October that it had made about 100,000 pairs of Ivanka Trump-branded shoes over the years.