China's Hebei Iron & Steel Group, its biggest steelmaker by output, accused the US of breaching WTO rules and said US protectionism is damaging the world steel trade, in a statement posted on its website on Thursday.

The US International Trade Commission (ITC) a week ago launched a probe into Chinese steel mills accused by US Steel Corp of stealing its secrets and conspiring to fix prices.

"The protectionist behaviour taken by the US based on purely groundless accusations by US Steel has seriously broken the WTO rules, distorted the normal world steel trade and damaged the essential interests of Chinese steel mills and US steel users," the statement said.

The Chinese steelmaker said it strongly opposed the probe and urged the US to understand the motivation of the complaint, assess the consequences brought by trade protectionism, respect objective facts and be careful when taking measures to cut trade.

Hebei Iron & Steel said it would appeal the probe, without saying to whom, and called on the Chinese government to take measures in line with WTO rules to maintain the legal interests of Chinese steel mills.

US Steel filed its original complaint a month ago, alleging it was a victim of a 2011 computer hacking incident that also prompted US federal cyber-espionage indictments against five Chinese military officials in 2014.

The ITC identified 40 Chinese steelmakers and distribution subsidiaries as respondents in its probe, including Baosteel Group, Hebei Iron and Steel, Wuhan Iron and Steel Co Ltd, Maanshan Iron and Steel Group, Anshan Iron and Steel Group and Jiangsu Shagang Group.

Baosteel, China's second-largest steelmaker and the world's fourth-largest, said in a statement the US was acting in breach of World Trade Organization (WTO) rules. It urged the Chinese government to take all necessary measures to ensure the sector receives fair treatment.

The China Iron & Steel Association (CISA) also said the government should take counter-measures against the US to support the steel industry. - Reuters