BEIJING - China is calling for fair treatment of Chinese companies participating in the US market.

"China hopes to see a more open market in the United States, and Chinese companies should be treated fairly in the market," Foreign Ministry spokesman Qin Gang said on Tuesday, responding to an American lawmakers' protest over a Chinese steel mill's investment in the US.

Qin said commercial cooperation between China and the US is beneficial to both.

Fifty US Congress members sent a letter to Treasury Secretary Timothy Geithner last week urging an investigation into cooperation plans between US-based Steel Development Company and China's Anshan Iron and Steel Group, also known as Ansteel.

The two signed an agreement in May for the construction of five steel plants in the US.

"We are deeply concerned that their direct investment in an American steel company threatens American jobs and our national security," the lawmakers said in the letter.

Ansteel was looking into the protest by the US lawmakers and was going to dispatch an executive to the US this week, Dow Jones Newswires reported.

A series of trade disputes have strained Sino-US ties in recent months, including the value of the Chinese currency and Google's spat with Beijing over censorship.

AFP contributed to the story.