An activist investigating a company that manufactures shoes for Ivanka Trump and other Western brands has been detained by police in China -- and two of his colleagues remain unaccounted for.

The activists were working undercover at two factories in southern China, according to China Labor Watch, the New York-based advocacy group for which they were carrying out the investigation. It lost contact with them on Saturday.

The news comes amid a broader crackdown in China on independent groups pushing for better labor rights in the world's second-largest economy.

One of the activists, Hua Haifeng, was detained by local police in Jiangxi province for "illegal surveillance," said Li Qiang, the head of China Labor Watch. The other two activists, Li Zhao and Su Heng, are missing, he said.

The activists took jobs between March and May at two factories run by Chinese shoe manufacturer Huajian Group -- one in Jiangxi and the other in Guangdong -- in order to investigate labor practices at the company, Li said.

Su, who was working at the factory in Jiangxi, found evidence of violations of workers' rights during the production of Ivanka Trump shoes, according to Li. They included employees working for as many as 18 hours a day for pay below the minimum wage, he said.

In a statement, Huajian denied the allegations of excessively long hours and low pay, but said it would look into another accusation by China Labor Watch that the company was illegally hiring students as interns.

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A spokeswoman for Marc Fisher Footwear, which licenses the Ivanka Trump brand for shoes, said the firm was unaware of the allegations and would look into them immediately. Two other shoe brands under the company's umbrella -- Marc Fisher and Kylie + Kendall -- were also being made at the Huajian factories, according to China Labor Watch.

Huajian makes millions of shoes for Western brands every year.

Other customers supplied by the two factories under investigation by China Labor Watch include Coach, Alain Delon, Nine West and Karl Lagerfeld, the advocacy group said. The companies didn't immediately respond to requests for comment.

Li said he is sending video footage shot by Su at the Jiangxi factory to Ivanka Trump's company.

"We appeal to President Trump, Ivanka Trump herself, and to her related brand company to advocate and press for the release our activists," he said.

There's nothing to suggest that Ivanka Trump has any connection to what happened to the China Labor Watch investigators. She resigned from her management role at the clothing and accessories company when she took a job in her father's administration, but she still owns part of the business.

A spokeswoman for Ivanka Trump's company declined to comment on the activists' case.

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Hua's wife, Deng Guilian, told CNN that she received a call Tuesday from police in Jiangxi telling her that her husband has been detained for conducting illegal surveillance and that she should expect to receive a formal notice in the mail soon.

Deng said the news came as a shock. And with family's only breadwinner now in police detention, she said she's worried about their financial situation.

The families of Li and Su couldn't be reached for comment.

Officials at the local police department in Jiangxi said they weren't aware of any cases involving China Labor Watch activists.

The detention and disappearances of the activists were first reported by The Associated Press.

The Chinese government has stepped up its clamping down on independent labor groups in recent years. Since 2015, dozens of activists and lawyers have been detained, arrested and harassed.

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"The government is taking a harsher line towards activists, especially those working for overseas organizations," said William Nee, China researcher at Amnesty International. "With the tightening economic situation, authorities could be looking to protect local manufacturing industries at all costs, even if that includes detaining activists bringing up labor issues."

But Li said it's the first time in 17 years of investigating labor rights violations in China that any of China Labor Watch's investigators had been detained by police.

"The accusation from Jiangxi police has no factual basis," he said.

-- Serenitie Wang, Serena Dong, Juila Horowitz, Yazhou Sun, Jethro Mullen and Felicia Wong contributed to this report.