BEIJING (Reuters) - China on Tuesday said it had detained labor rights activists who scrutinized conditions at a Chinese company making Ivanka Trump-branded shoes, rebuffing a call from the U.S State Department for the release of the three men.

A soldier from honour guards holds a red flag during a welcoming ceremony for Vietnam's President Tran Dai Quang (not pictured) outside the Great Hall of the People, in Beijing, China May 11, 2017. REUTERS/Jason Lee

The remarks were the first official confirmation of last month’s statement by China Labor Watch, a New York-based advocacy group, that one activist had been arrested and two were missing after studying conditions at factories that produce shoes for the daughter of U.S. President Donald Trump and other Western brands.

Chinese Foreign Ministry spokeswoman Hua Chunying said the activists were detained and investigated on suspicion of interfering with a company’s “normal operation and production activities”, and the illegal use of “professional surveillance equipment”.

The case was being handled in accordance with the law and no country had the right to interfere in the judicial process, she told a daily news briefing in the Chinese capital.

On Monday, a spokeswoman for the U.S. State Department expressed concern over reports that Chinese authorities had detained labor rights activist Hua Haifeng and that two other labor activists – Su Heng and Li Zhao – were also missing and presumed to have been detained.

“We urge China to release them immediately and otherwise afford them the judicial and fair trial protections to which they are entitled,” said the spokeswoman, Alicia Edwards.

Labour activists have helped U.S. companies understand conditions in their China supply chains, “which can be essential in fulfilling their own responsibilities and holding Chinese manufacturers accountable under Chinese labor laws,” she added.

China Labor Watch said Hua Haifeng was arrested in southeastern Jiangxi province on suspicion of illegally using eavesdropping equipment.

He and Li Zhao had worked covertly at a shoe factory owned by the Huajian Group in the city of Dongguan, in southern Guangdong province. The third investigator, Su Heng, had worked at a related factory in the Jiangxi city of Ganzhou, but had been incommunicado after May 27.

Both factories produced Ivanka Trump-branded shoes, China Labor Watch said. The investigators had discovered evidence that workers’ rights had been violated, it said.

The Ivanka Trump brand, the White House and Ivanka Trump’s lawyer, Jamie Gorelick, have not responded to requests for comment on the case.

The case comes at a time of sustained pressure on labor activists in China amid a crackdown on civil society under President Xi Jinping.

In recent years, many labor rights activists have reported being intimidated and harassed, temporarily detained, or having had their movements restricted.

Rights group Amnesty International has called for the release of the three if they were held only for investigating possible labor abuses at the factories.