SHANGHAI (Reuters) -China's "Big Four" state banks said late on Wednesday that their state parent, Central Huijin Investment Ltd, had bought the lenders' Shanghai-traded shares, and plans to further increase holdings within the next six months.

The announcements by Bank Of China, Agricultural Bank of China (AgBank), Industrial and Commercial Bank of China (ICBC) and China Construction Bank (CCB) come as Beijing is struggling to revive confidence in China's flagging stock market.

China's blue-chip CSI300 Index fell to 11-month lows this week despite a slew of supportive measures by the government.

The four lenders said in separate exchange filings that controlling shareholder Huijin on Wednesday bought their shares via the trading system of the Shanghai Stock Exchange, and intends to continue to increase its shareholding in the banks by acquiring shares on the secondary market.

Beijing-based Huijin, mandated to make equity investments in major state-owned financial institutions on behalf of the central government, bought 24.9 million shares in Bank of China, and 37.3 million shares in AgBank, according to the filings.

Huijin also increased stakes in ICBC and CCB by 27.6 million shares, and 18.4 million shares, respectively.

(Reporting by Shanghai newsroom, Editing by William Maclean)