China's central bank is tightening rules on interbank bond market trading by ordering all transactions to be conducted through the National Interbank Funding Center as it seeks to boost transparency. Transactions including forward deals and repurchases can't be reversed or changed once agreed between the two parties, the People's Bank of China said in a statement posted on its website on Tuesday. Clearing agencies should not engage in settling trades outside the interbank market, according to the statement. Alterations to bond ownership, such as inheritance that are not related to trading, must be supported by legal documents explaining the nature of the transaction, it added. Chinese authorities are seeking to clean up the $3.8 trillion market and encourage companies to raise funds through bonds rather than relying on bank lending. The State Council, or China's cabinet, approved trading of government debt futures for the first time in 18 years to allow investors to hedge risk, the China Securities Regulatory Commission said this month.