(Bloomberg) —China told the U.S. to mind its own business, rebuffing a call for the release of three labor activists detained while probing labor conditions at a factory that manufactured Ivanka Trump brand shoes.

“No country can interfere in China’s sovereign and judicial independence,” foreign ministry spokeswoman Hua Chunying said in response to a question at a regular briefing on Tuesday, adding the case is an “internal matter.”

It’s the first time the two countries had a diplomatic exchange on the case. The suspects are accused of using spying and monitoring equipment, and the investigation is ongoing, Hua said.

New York-based China Labor Watch said the three were working undercover to investigate a factory operated by shoe manufacturer Huajian Group in Ganzhou city in the southeast province of Jiangxi when they went missing late last month. Huajian’s facility is one of 15 factories that have made products for the brand founded by the daughter of U.S. President Donald Trump that the labor rights group said it has been investigating in the past year.

Earlier on Tuesday, the U.S. State Department weighed in on the detentions. “We are concerned by reports that Chinese authorities have detained labor-rights activist Hua Haifeng and that two other labor activists — Su Heng and Li Zhao — are also missing and presumed to be detained,” said Alicia Edwards, a spokeswoman for the Bureau of East Asian and Pacific Affairs. “We urge China to release them immediately and otherwise afford them the judicial and fair trial protections to which they are entitled.”

“I think the State Department has made it very clear the U.S. position on that,” said White House spokesman Sean Spicer when asked about the detentions during his daily briefing with reporters. “We’ll continue to exert the proper diplomatic pressure on that.”

Labor Conditions Investigated

All three men have been detained for illegal eavesdropping, Wen Yu, a lawyer representing Hua, said. “It’s a new law,” Wen said. “We don’t know how it’s going to proceed or how it will be used.”

In a letter sent to Ivanka Trump in April, China Labor Watch alleged a number of labor violations at two unnamed factories that supplied the Ivanka Trump brand, including that employees were forced to work at least 12 1/2 hours a day for wages below China’s legal minimum — a monthly salary equivalent to about a dollar an hour.

Ivanka Trump shoes are manufactured under license by  Marc Fisher Footwear Co., which has said it was looking into the allegations. Representatives for Ivanka Trump’s brand declined to comment. Marc Fisher and Huajian representatives couldn’t be reached for comment Tuesday.

Huajian said last week it stopped producing Ivanka Trump shoes in March because there were no outstanding orders. The company has denied China Labor Watch allegations that it underpaid employees and that workers worked excessive hours.

Wen was able to see his client, Hua, Tuesday afternoon and has requested bail for him, the lawyer said. Su Heng’s attorney met with Su Monday, according to Wen.

The undercover investigators were discovered last month, according to Wen. Hua then tried to enter Hong Kong but was denied entry. He returned to Ganzhou and was picked up May 28, Wen said. The three are being detained in the same Ganzhou facility, according to Wen. Hua will be detained until June 28, the 30-day standard detention for police to determine whether to formally arrest a suspect, Wen said.

High-Level Handling

The case is being handled by the national security unit of the public security bureau, suggesting it’s being managed at a high level, Wen said.

Hua was supposed to deliver evidence of violations, including images and videos, to China Labor Watch in Hong Kong last month, according to Li Qiang, the founder of the labor rights group. Instead, he was barred from leaving the country without being given a reason, Li said.

Calls to the Ganzhou Public Security Bureau went unanswered Tuesday.

The detentions come as China has increased regulatory restrictions on labor rights advocates and other non-governmental organizations as well as imposing a broader clampdown on social discourse concerning politically sensitive topics. China has long protested comments from foreign governments on matters it considers to be purely domestic affairs, particularly legal proceedings and issues of human rights.

Aggressive Campaign

“The trio appear to be the latest to fall foul of the Chinese authorities’ aggressive campaign against human rights activists who have any ties to overseas organizations, using the pretense of ‘national security,’” said William Nee, China researcher at human rights organization Amnesty International.

It’s the first time in 17 years that activists working for the nonprofit workers-advocacy group have been detained by the Chinese police, Li said. China Labor Watch previously identified labor violations at a Chinese toymaker used by Walt Disney Co., leading the entertainment giant to sever ties with the factory. It has also investigated plants used by Apple Inc.

“Labor activists have been instrumental in helping American companies understand the conditions in their supply chains, which can be essential in fulfilling their own responsibilities and holding Chinese manufacturers accountable under Chinese labor laws,” Edwards said.

–With assistance from Stephanie Wong, Keith Zhai, Lindsey Rupp and Natasha Khan.To contact Bloomberg News staff for this story: Nick Wadhams in Washington at nwadhams@bloomberg.net ;Rachel Chang in Shanghai at wchang98@bloomberg.net ;Peter Martin in Beijing at pmartin138@bloomberg.net To contact the editors responsible for this story: Bill Faries at wfaries@bloomberg.net ;K. Oanh Ha at oha3@bloomberg.net Michael B. Marois, Ken Wills