China has defended the detention of three activists who were investigating working conditions in factories that make shoes for companies owned by President Trump's daughter Ivanka.
It says the trio had used illegal surveillance equipment, interfering with the operation of the factories.
It is the first time the government has confirmed their detention.
The US state department earlier called for their release, saying they should otherwise receive a fair trial.
It argues that labour activists play a vital role in helping US firms understand working conditions at supply companies.
The three were detained last month while probing alleged abuses at a supplier for Ivanka Trump's shoe brand.
They are reported to have been preparing a report listing a series of serious shortcomings, including verbal abuse and violations of the rights of female workers.
Chinese foreign ministry spokeswoman Hua Chunying said that the activists were being investigated on suspicion of using illegal "professional surveillance equipment".
The case was being dealt with according to the law and no foreign country had the authority to meddle in China's judicial process, she told a news briefing in Beijing.
Hua Haifeng was working undercover for the US-based China Labor Watch (CLW), which said he had witnessed examples of forced overtime and wage violations at Huajian Group factories.
His lawyer told the New York Post newspaper that all three men were being held at the Ganzhou City Detention Center in Jiangxi province and that he had been blocked from seeing his client through most of Monday.
The Huajian Group also makes products for other brands including Karl Lagerfeld, Coach and Nine West, CLW has said.
The company has denied allegations that staff have to work excessively long hours for poor pay, and insists that it no longer manufactures shoes on behalf of Ivanka Trump.
Amnesty International has also demanded the release of the trio.
CLW said that suspicions that Mr Hua had illegally used eavesdropping equipment had "no factual basis".
The other two men - named as Li Zhao and Su Heng - are reported to have been working undercover at Huajian factories in Dongguan in Guangdong province and in Ganzhou.
China has seen frequent crackdowns on labour rights activists and rights lawyers in recent years, with campaigners being harassed, arrested or jailed.