The United States Wednesday called on China to explain why it appeared to be blocking websites of US firms, as Washington took the first steps to bring the case to the World Trade Organization.

"US businesses... have expressed concerns regarding the adverse business impacts from periodic disruptions to the availability of their websites in China," the US Trade Representative said in a statement.

The request was made under world trade rules which require members to provide information about potential barriers to trade.

The nearly 50-question filing asks who is responsible for blocking websites, why and when it takes place.

"The United States would like to better understand Chinas rules governing website blocking so that service suppliers based outside of China may adopt appropriate policies to avoid encountering this problem."

Although the request touched on the sensitive issue of Chinese Internet censorship for political reasons, the US Trade Representative said it would focus on trade issues.

"While the United States believes that the best Internet policy is to encourage the free flow of information globally, the United States's WTO request relates specifically to the commercial and trade impact of the Internet disruptions," it said.

But the request covers a range of issues that span the political and economic gamut and could spark controversy, such as "who or what ministry is responsible for determining if and when a foreign website should be blocked in China?"

Tempers have already flared over Google's services in China.

Google said in March last year it would no longer bow to government censors and effectively shut down its Chinese search engine, re-routing mainland users to its uncensored website in Hong Kong.

The Computer and Communications Industry Association hailed the USTR move, with president Ed Black calling it a "huge first step in the process of holding China accountable for imposing unlawful barriers to international trade."

"At a time when the US is experiencing a huge trade deficit, aggressively fighting against illegal trade restrictions in such a key export industry is a no-brainer," Black said in a statement.

The move comes as the White House comes under pressure to be firmer with China about what lawmakers say are unfair trade policies.

A USTR spokesperson said the United States was not trying to pick a fight: "Our goal is to work cooperatively with China on this issue."

The Senate approved a bill last week to punish Beijing for its alleged currency manipulation, drawing a furious reaction from Beijing as well as complaints from the White House, which is known to oppose the measure as written.

The Republican leadership in the House of Representatives has so far resisted pressure to pass the bill, but pressure is mounting for lawmakers to act.

A key US congressional panel plans to put the White House in the hot seat at an October 25 hearing to study action against China's trade stance.

"China's distorting trade policies are deeply troubling and cannot be allowed to stand. Its practices are costing US jobs," House Ways and Means Committee Chairman Dave Camp said in a statement announcing the session.

"I look forward to hearing the administration's plan for addressing China's persistent barriers to US exports and investment and exploring what should be done to ensure American employers and workers are treated fairly," he said.