China's central bank auctioned 15 billion yuan (US$2.2 billion) of three-year bills with a yield of 2.75 percent in its open market operations Thursday, a sign of tightening liquidity, analysts said.

The People's Bank of China (PBOC), the country's central bank, stopped issuing three-year bills in June 2008 to reverse the economic slowdown in the middle of the global financial crisis.

In its open market operations Thursday, the PBOC also auctioned 75 billion yuan (US$11 billion) of three-month bills with a yield of 1.4088 percent.

The central bank drained another 60 billion yuan (US$8.8 billion) Thursday through 91-day bond repurchase agreements.

Market analysts said though the volume of the bills was not so big, the indications of the central bank further tightening liquidity had been on the increase.

Liu Yuhui, an economist with the Chinese Academy of Social Sciences, said the three-year bills could lock up more liquidity on top of the current issuance of three-month and one-year bills.

"The central bank's move is a sign of further liquidity tightening and reflects a tightening trend of credit policy," Liu said.

Sources close to the central bank said the three-year bills would be issued every two weeks.

Analysts said the PBCO's resumption of the three-year bills issuance was closely related to China's strong economic data in the first quarter and reflected the timely control of monetary and credit policy.

A total of 180 billion yuan in central bank bills and bond repurchase agreements will mature this week, leaving the central bank to soak up liquidity excesses from the market.