The four employees of mining giant Rio Tinto charged with taking bribes and stealing commercial secrets appealed for lenient punishment in court on the second day of their trial after pleading guilty to bribery charges, their lawyers said on Tuesday.

Zhang Peihong, a lawyer for one of the accused, said Australian Stern Hu and three Chinese employees of the world's second largest miner asked for mitigated punishment from the court after they admitted to taking bribes and returned part of the money.

The four altogether are charged with accepting bribes exceeding 86 million yuan ($12.6 million) according to the court indictment, but the defendants all contested the amount as specified by the prosecutor, according to their lawyers.

Tom Connor, the Australian consul general who attended the hearing on bribery charges, told reporters on Monday that Stern Hu was accused of taking bribes worth 1 million yuan and $790,000. He also said Hu "acknowledged the truth of some of the bribery charges" against him.

If convicted, Hu and the other three defendants - Liu Caikui, Ge Minqiang and Wang Yong - would face jail terms from 5 to 15 years.

But Tao Wuping, who is representing Liu, said "part of the charge (against Liu) should not constitute a crime" and that he would appeal for a lighter sentence of less than five years for the defendant.

The final jail terms for the four, however, remain unclear as they are also charged with industrial espionage which, if convicted, has a maximum penalty of seven years. The cap for fixed-term imprisonment is 20 years in China.

The trial entered a closed session on Tuesday afternoon to consider the charges of infringing commercial secrets. The indictment alleges the four used improper means to obtain Chinese commercial secrets from steel makers.

Earlier media reports said a senior executive of major steel maker Shougang Group was detained on July 7 last year in Beijing for alleged "commercial crimes" shortly after the detention of Stern Hu. A high-ranking employee from another steel maker, Laigang Group in Shandong province, was also under investigation for "providing secrets to Hu".

The reports also said the commercial secrets obtained from the country's steel industry have been used as a bargaining chip to drive up the price that China pays for its iron ore imports.

Australia has sought more transparency regarding the commercial confidentiality charge.

China's Foreign Ministry spokesman Qin Gang reiterated on Tuesday that court proceedings related to commercial secrets are a closed affair according to the Chinese law and that the consular agreement should not override the law.

He also said the Chinese government would update the Australian side on the latest developments.

The Rio case, together with Google's closing of its search engine on the Chinese mainland, have been widely seen by foreign media as a sign that foreign business sentiment is souring against China, where legal boundaries are blamed as vague and courts for being closely tied to the state.

However, Bo Haibao, director of the School of Politics and Law at Shanghai Finance University, said there needs to be greater respect of Chinese law.

"China has made great progress in improving its legal system, but there are still improvements to be made," he said.