The US is leaning on China to explain why it blocks so many foreign companies from providing internet services in the market.
The WTO's US Ambassador called on his Chinese counterpart to disclose the criteria for blocking access to a foreign website, and details of how those criteria are decided on, Reutersreported.
The letter states that a number of service suppliers are concerned about the impact web blocks are having on their ability to break into the world's largest market.
China blocks websites including Facebook, Twitter and YouTube as part of its internet censorship efforts. The country is also rumored to be planning greater restrictions on what can be said online.
But the Ambassador's letter makes clear that the US is not challenging China's restrictions on free speech online.
It also stops short of threatening to take any action with the WTO, but commentators have suggested that the internet restrictions are in violation of WTO agreements to which China is a signatory.
Web censorship may have taken a heavy toll on the domestic internet industry as well – the number of domestic websitesshrank 41% between 2009 and 2010, a government think-tank estimated in June. But the same organisation denied that political censorship was to blame.
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