China stepped up its campaign against the Anglo-Australian mining giant Rio Tinto on Wednesday by saying the company had bribed virtually every one of China's big steel makers. The stunning allegation, published on the front page of China Daily, the country's official English-language newspaper, ratcheted up the stakes in a case that has already rocked the country's steel industry and strained relations between China and Australia. In its report, China Daily, which regularly publishes government messages, said Rio Tinto bribed executives from 16 Chinese steel companies, all members of the China Iron & Steel Association, to get access to industry data. The paper attributed the allegation to an unidentified 'industry insider' and did not identify the 16 companies. The paper said executives from the 16 steel companies had recently taken part in failed negotiations with Rio Tinto over iron ore prices. The report comes more than a week after Chinese authorities detained four Rio Tinto employees, including one Australian national, on suspicions of espionage, stealing state secrets and harming the nation's economic interests and security. A Chinese government-controlled Web site said last week that Rio employees had bribed Chinese steel executives to gain access to government documents that could have given Rio Tinto an edge in its annual iron ore negotiations with China's state-owned steel mills. But Wednesday's report is the first time a government publication has alleged that many of the country's biggest steel companies may have been involved in the case. Rio Tinto has expressed shock at the allegations and said that the company has strict ethics policies that forbid bribery. On Wednesday, a spokesman for the company declined comment. Also on Wednesday, several large Chinese steel companies, including BaoSteel, strongly denied they were under investigation in the Rio Tinto case. 'BaoSteel has no executive cooperating with the government investigation or being investigated,' said Meng Haibiao, a company spokesman. One steel company, Jinan Steel, called the China Daily report 'fake news,' insisting no company executive was under investigation. But at Laiwu Iron & Steel, a spokesman who gave his name only as Liu, said one of the company's shipping executives was cooperating with the Rio investigation. In the past week, China has questioned or detained at least 10 steel executives, including shipping agents, traders and members of the steel association, according to the state-run news media and interviews with industry officials. An executive at Shougang Steel, a big steel maker, has been detained in the case. In Australia, government officials have criticized the detention of the four Rio Tinto employees and suggested that Beijing is retaliating against Rio Tinto for scuttling a deal last month that would have given a Chinese state-owned company a large stake in the mining giant. The Australian prime minister, Kevin Rudd, said Wednesday that he was concerned about the state secrets law case against Rio employees. According to Reuters, Rudd told reporters, 'A range of foreign governments and corporations will be watching this case with interest and will be watching it very closely. And they will be drawing their own conclusions as to how it is conducted.' In Beijing, Commerce Secretary Gary Locke of the United States also spoke out, saying he would speak to some of China's top leaders about the Rio Tinto case because Americans doing business in here are concerned about transparency and fairness. A spokesman for China's Foreign Ministry declined comment Wednesday. Although Beijing says the Rio Tinto case involves state secrets, that has not stopped state-run newspapers from printing details about it. But many of the articles have also described rampant corruption in the trading of iron ore. More than a dozen industry officials, steel analysts and traders interviewed this week also said China's iron ore trade was plagued by corruption. Experts blame China's two-tiered system for purchasing iron ore. While big steel mills are allowed to negotiate long-term, fixed-price contracts, most small and medium-sized steel mills are supposed to buy from the more volatile, open market -- or what is called the spot market. The system, analysts say, has created huge arbitrage opportunities, allowing big steel mills with fixed contracts to buy far more supplies than they need and then profitably sell excess supplies to smaller mills on the black market. 'There's a huge underground arbitrage market in China,' said Ren Qiang, general manager at Zibo Antai Import and Export Company, which deals in the iron ore trade. Small mills have long complained about what they see as an unfair system. The practice violates industry regulations, as well as the fixed contract agreements signed with Rio Tinto and other mining companies, analysts say. The government has repeatedly warned about this type of speculative trading, most recently in a regulation released in February. But regulators rarely enforced the rules in 2007 and early 2008, when iron ore prices were skyrocketing, experts say. Analysts say the two-tier system may have also created friction between big mining companies, like Rio Tinto, and the government-controlled China's Iron & Steel Association, which represents large mills in annual fixed contract negotiations. Analysts say by locking in low fixed contract prices, association members could then trade or sell iron ore on the black market for additional profits. But the world's big iron ore producers, like Rio Tinto, have battled with the association over pricing. When negotiations broke down in 2008, China's steel association urged Chinese mills and traders to boycott Rio Tinto's spot market iron ore. Rio Tinto, in turn criticized the association for using aggressive tactics. The two sides failed to reach an agreement last month on prices for this year's contracts. One of the lead negotiators for Rio Tinto was Stern Hu, an Australian national who was detained on July 5 for what Beijing says was stealing state secrets and harming China's economists interests. 