SHANGHAI, Aug 27 (Reuters) - China shares rose on Friday after investors took comfort in the central bank’s biggest weekly cash injection into the banking system since February, and as authorities urged more financial support for retail and trading firms. The blue-chip CSI300 index rose 0.5%, to 4,827.04, while the Shanghai Composite Index gained 0.6% to 3,522.16 points. ** The Shanghai Composite Index logged a weekly gain of 2.8%, the biggest in three months, while the CSI300 index finished up 1.2% for the week. ** China’s central bank injected 50 billion yuan ($7.71 billion) through seven-day reverse repos into the banking system on Friday for the third straight session. The weekly total injection is a net 120 billion yuan, the largest since the start of February. ** Financial institutions should do more to help retail, catering, and trading firms in the wake of recent domestic coronavirus outbreaks, the commerce ministry said on Thursday. ** Profits at China’s industrial firms in July grew at their slowest clip this year, adding to a recent batch of indicators highlighting loss of momentum in the country. ** “With the rising risk of a growth slowdown and the lack of flexibility in some key existing tightening measures, we believe the probability of an RRR cut is on the rise in the near term,” Nomura analysts said in a note, referring to the banks’ reserve requirement ratio. ** Investors are waiting for the Fed’s Jackson Hole symposium on Friday for clues on the timing of a tapering of monetary stimulus, a move that could drain capital from emerging markets. ** Shares of carmakers led the gains, with the sub-index up 3.8%, after Ministry of Industry and Information Technology of China organised a meeting to support accelerating the development of new energy vehicles. ** The insurance sub-index and the non-ferrous metals sub-index gained 2.1% and 2.6%, respectively. ** The media sub-index dropped 1.4%, as China cracked down on what it described as a “chaotic” celebrity fan culture on Friday after a series of scandals involving artists. ** A sub-index tracking defence stocks fell 1.6%. Reporting by Shanghai Newsroom Editing by Tomasz Janowski Our Standards: The Thomson Reuters Trust Principles.