(The opinions expressed here are those of the author, a columnist for Reuters)

LONDON, March 22 (Reuters) - For Turkish firms exporting marble and other stone to an increasingly lucrative Chinese market, the Xiamen International Stone Fair should have been the business highlight of the year. Instead, the arrest and detention of four Turkish executives for tax evasion sent scores of others fleeing, grabbing any flight they could from mainland China for fear they might be next.

Turkey's Hurriyet Daily News reported widespread panic here among attendees. One vowed never to return to China, saying he was lucky to find a place on the first plane out.

It appeared here to be a deliberate and brutal reminder by Beijing of the economic costs of Turkey's February criticism here of China for the treatment of its Muslim Uighur minority. With a U.N. panel last year here accusing Beijing of interning up to a million Uighurs in so-called “re-education camps”, it’s a topic many in the rest of the world feel they can no longer quietly ignore.

Nor, however, is it something most countries – or businesses – want to dominate their relations with a growing superpower. It’s a dilemma made more challenging still by Beijing’s apparent complete unwillingness to accept criticism on the issue, coupled with the very real difficulty of knowing what is truly happening to Uighurs in the Northwest province of Xinjiang.

Earlier this month, U.N. human rights chief Michelle Bachelet made her second request here in six months for access to the region – but few believe Beijing has any real intention of allowing a detailed outside investigation. Those who watch the region closely, however, say things are getting worse.

Last week here, the United States made its most overt criticism of Beijing so far on the issue, saying abuses against China's Muslim minorities were on a scale not "seen since the 1930s". Beijing’s campaign of internment against Uighurs, ethnic Kazakhs and other Muslim minorities has intensified significantly over the last year, it said, with numbers now reportedly detained ranging from 800,000 to two million, double the upper number estimated by the United Nations last year.

Another academic estimated here the potential number of internees at 1.5 million, roughly a sixth of China’s Uighur population.

HUMAN RIGHTS

Speaking at the State Department’s annual review of human rights, Secretary of State Mike Pompeo said Beijing was now in a “league of its own when it came to abuses”. What is much less clear, however, is how willing Washington will be to let these concerns interfere with ongoing trade negotiations with China. Ultimately, many human rights activists fear the fate of China’s Uighurs may become a strictly secondary concern when pitted against more hard-nosed security and economic considerations.

Foreign business leaders, politicians, journalists and academics all privately express their worries about publicly commenting on the Uighur interments, fearing the loss of access to China on which careers and business models often depend.

The Chinese government, which initially refused to acknowledge the existence of such camps at all, allowed selected diplomats and journalists – including Reuters – extremely limited and curated access to a camp earlier this year. Chinese officials say they are “vocational training centres”, with one saying “absurd preachings” by Islamist extremists had turned some in the region into “murderous devils”.

A handful of Chinese Uighurs are believed to have fought for Islamic State and associated groups in Afghanistan, Iraq, and beyond, and suspected Islamist militants are believed to be behind a small number of attacks in China, including several 2013-14 assaults in Beijing, Kunming and Urumqi that killed dozens. None of that, however, comes close to justifying the scale of the current clampdown, human rights activists say.

TRADE WAR

Washington has become increasingly outspoken over the Uighurs as its trade war with Beijing intensifies, with U.S. officials saying they are considering targeted sanctions against officials involved in the crackdown. European states, in contrast, have remained largely silent. This week, a consortium of human rights groups, including Amnesty International and Human Rights Watch, criticized what they said here were “missed opportunities” to call China out on its abuses. They called for the European External Action Service – essentially the foreign ministry of the European Commission – to follow Washington in publishing its own frank assessment.

Elsewhere, silence – even outright collaboration in Beijing’s actions – appears to be spreading, an apparent sign of China’s diplomatic and economic clout. In September, Pakistan became here&IR=T one of the first Muslim countries to openly criticize Beijing for the crackdown, saying it risked further worsening extremism. Since then, however, it has declined to repeat such views – and Pakistan’s own tiny Uighur population complain of mounting harassment by authorities, viewed by many as a sign Pakistan too has succumbed to Chinese pressure.

At the U.N. Human Rights Council last month here, Britain was the only country willing to join Turkey in raising the Uighur issue at all. Last month, Saudi Crown Prince Mohammed bin Salman issued an outspoken statement in support of China’s clampdown as he toured mainland China. Several Muslim countries including Egypt have deported visiting Uighur students back to China, apparently under government pressure. In Kazakhstan, authorities earlier this month here arrested a prominent Xinjiang-born activist who had been an outspoken critic of China’s clampdown.

With his own record of oppression against the Kurds and a brutal crackdown following a failed 2016 coup, Turkey’s President Tayyip Erdogan is an unexpected champion for an oppressed minority, even given historically strong Uighur-Turkish links. Perhaps that is why China seems so furious, keen to impose an economic and diplomatic price that will deter other leaders from doing the same.

The awkward truth is that there are serious limits to what the outside world can do to help the Uighurs. But that doesn’t mean the debate should be silenced – because a world in which vast numbers can be herded into camps without consequences could become a very dark place indeed. ** Peter Apps is a writer on international affairs, globalisation, conflict and other issues. He is the founder and executive director of the Project for Study of the 21st Century; PS21, a non-national, non-partisan, non-ideological think tank. Paralysed by a war-zone car smash in 2006, he also blogs about his disability and other topics. He was previously a reporter for Reuters and continues to be paid by Thomson Reuters. Since 2016, he has been a member of the British Army Reserve and the UK Labour Party, and is an active fundraiser for the party. (Editing by Giles Elgood)