Islamabad (AFP) - Every autumn on the mountainous Karakoram Highway, part of the ancient Silk Road, groups of Pakistani merchants living in China's far west would wave goodbye to their Chinese wives and cross the border to spend winter in their home country.

As the snow piled high, the men would stay in touch with their families by phone, longing for the spring thaw that would allow them to be reunited in Xinjiang.

But last year many of their calls suddenly went unanswered.

Their families, they learned, had disappeared into a growing network of shadowy "reeducation centres" that have swept up the region's Uighur Muslim minority over fears of Islamic militancy crossing the border from Pakistan.

"My wife and kids were taken away by the Chinese authorities in March last year and I haven't heard from them since," said Iqbal, a Pakistani businessman who declined to give his surname over concern about his family's safety.

Last July, he headed to China to find them, but was turned away at the border. Authorities "said my wife was in 'training' and the government was taking care of my kids", he told AFP.

"I begged them to let me talk to my daughters, but they refused."

Iqbal is one of dozens of merchants from Gilgit-Baltistan who return to Pakistan for visa reasons or to run their businesses and have been unable to contact their Uighur families living in China, according to Javed Hussain, a member of the local assembly for the Pakistani region that borders Xinjiang.

Earlier this month, the delegates passed a unanimous resolution protesting the "illegal detention" of the men's families.

"The Chinese authorities should at least allow the men to meet their wives and children," Hussain said.

"China is our friend and this incident will leave a bad taste."

China's foreign ministry said that the "two sides are maintaining communication about problems related to interactions between both countries' people", while Pakistan's said the issue was being "actively discussed with the government of China".

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- 'Eliminating extremism' -

Like many of the men, Iqbal's family lived in Kashgar, an ancient city along the China-Pakistan Economic Corridor (CPEC), a trade route connecting China's far west to the Arabian Sea port of Gwadar.

In recent years, China has heavily pushed its relationship with Pakistan, investing tens of billions of dollars in CPEC infrastructure projects in the country, and Beijing has upgraded the treacherous mountain road connecting Gilgit-Baltistan to Xinjiang.

But China has had difficulty reconciling its desire for development with fears that Uighur separatists will import violence from Pakistan.

Chinese authorities have long linked their crackdown on Xinjiang's Muslims to international counter-terrorism, arguing that separatists are bent on joining foreign extremists like Al-Qaeda.

Uighurs have been tied to mass stabbings and bombings that left dozens dead in recent years across the country. Riots and clashes with the government killed hundreds more.

Over the past year, China has turned to increasingly drastic methods to eliminate what it describes as the "three forces": terrorists, religious extremists and separatists.

In 2017, the government flooded Xinjiang with tens of thousands of security personnel, with police stations on nearly every block in urban areas and tough regulations to "eliminate extremism".

This included the increased use of compulsory "reeducation" for anyone suspected of harbouring separatist sympathies.

- 'Threat' from Pakistan -

Iqbal and the other Pakistani men believe their wives -- and even business associates -- have been targeted because they received calls and messages from Pakistan.

"Any communication from Pakistan is considered a threat," said Qurban, a businessman who has worked in Kashgar for over 30 years.

"One of my employees, a Uighur, was picked up two years back just because he was in touch with me when I went to Pakistan."

Chinese authorities have denied the existence of reeducation centres.

But regulations against extremism adopted by Xinjiang last March call for authorities to step up political reeducation.

In Kashgar alone, more than 120,000 people -- about three percent of the area's population -- were being held in the facilities in January, according to Radio Free Asia.

An AFP review of state media reports and government documents verified the existence of at least 30 such centres and almost 4,000 cases of people being sent to them.

Regulations posted on a local website in Xinjiang's Hejing county explained that even minor transgressions of strict religious regulations can be punished with up to three months in a centre.

Ali, a businessman who lost contact with his wife in December, said she had been taken by authorities to do a "sort of training where they teach them about Communism and prepare them to be patriotic citizens".

"My wife told me that Chinese police had come to her house and asked her about the calls from Pakistan and asked her to explain her links with ETIM," said Ali, referring to the East Turkestan Islamic Movement, a militant group China has accused of attempting to foment Uighur separatism.

He plans to cross the border in May to find his family, but has been told his children are in the custody of the Chinese government and doesn't know if he will see them again.

"They never tell you anything, they just say your family will come back to you when they finish their training."