The United States is pressing China to explain why its national firewall blocks so many U.S. companies from providing their services via the Internet, according to a letter obtained on Wednesday that is another sign of growing trade tension between the world's two largest economies.

Having a presence on the Internet that is visible in China is increasingly a critical element for service suppliers aiming to reach Chinese consumers and business, U.S. Ambassador to the World Trade Organization Michael Punke said in a letter to his Chinese counterpart on Monday.

Some companies based outside of China have faced challenges offering their services to Chinese customers when their websites are blocked by China's national firewall, Punke said, adding that the U.S. government has heard concerns from a number of service suppliers about the problem.

China bans numerous websites, including Facebook, Twitter and YouTube and some foreign media outlets, fearing the uncensored sharing of images and information could cause social instability and harm national security.

The so-called Great Firewall of China can also cause blockage or slow loading of websites not subject to the ban, and is the source of widespread frustration to users in China.

Punke's letter posed detailed questions on Chinese policy on the Internet, asking how China defines illegal information and requesting that Beijing spell out how it implements the bans and what recourse is available to targeted websites.

What are the guidelines and criteria for blocking access to foreign websites? How often are these guidelines and criteria changed or published? Where are these guidelines published? Are they made public in advance of their implementation? read one multi-part question to China's envoy.

The letter stopped short of threatening to bring legal action against China at the WTO.

Ed Black, president of the Washington-based Computer and Industry Communications Association hailed the action as a huge first step in the process of holding China accountable for imposing unlawful barriers to international trade.

Chinese Internet restrictions are not transparent, lack procedures that provide parties due process and are often applied more broadly and arbitrarily against non-Chinese companies -- all of which are violations of WTO law that has been agreed to by China, Black said.

Punke took pains to say the United States was not challenging Beijing's restrictions on free speech on the Internet even though Washington believes that economic and social development of the Internet globally is best served by policies that encourage the free flow of information and prioritize individual empowerment and responsibility.

Punke noted that a June 2010 Chinese government white paper on its internet policy provided helpful background, but also led to additional questions.

Further information regarding the laws, regulations and other measures establishing and implementing the Chinese government's polices covering services that can be provided over the Internet would help U.S. companies better serve the Chinese market, he said.

(Reporting by Doug Palmer; editing by Will Dunham and Mohammad Zargham)