BEIJING, Dec. 17 -- Central Huijin Investment Ltd, a state-owned investment company, has increased its stake in China's four big banks, in a move to boost the flagging stock market.

The banks filed statements to the Shanghai Stock Exchange on Monday evening.

Industrial and Commercial Bank of China said Central Huijin had purchased 175 million A-shares of the bank by Dec. 12, about 0.05 percent of the institution's capital stock. Huijin's share in the bank is now 35.36 percent.

China Construction Bank said Huijin had purchased 103 million A-shares of its bank by Dec. 12, accounting for 0.04 percent of its capital stock. Huijin's share in the bank is now 57.26 percent.

Bank of China said Huijin had bought 113 million of its A-shares, or 0.04 percent of the bank's capital stock. Huijin's share in the bank has increased to 67.75 percent.

Agricultural Bank of China said Huijin had purchased 179 million of its A-shares, or 0.06 percent of the bank's capital stock. Huijin's share in the bank has now reached 40.28 percent.

Set up in 2003, Central Huijin holds the Chinese government's stakes in commercial banks, securities companies, insurance firms and other institutions.

The company's moves have been considered by many as a way to shore up market confidence.

On Monday, the benchmark Shanghai Composite Index dropped 1.60 percent and the Shenzhen Component Index lost 1.86 percent.

HSBC released its flash manufacturing purchasing managers' index for December on Monday, which eased to 50.5, marking a three-month low in growth rate.