The Federal Reserve Bank of New York said Wednesday it would buy $18 billion in Treasury securities in operations to take place through mid-September.

The announcement by the New York Fed Wednesday comes after Tuesday’s gathering of the Federal Open Market Committee. There, policy makers announced their intention to keep the Fed balance sheet from shrinking, saying they would reinvest the proceeds of maturing mortgage securities back into the Treasury market, seeking to keep the central bank’s holdings right around $2 trillion.

The Fed’s action has been attended by controversy, with many in financial markets seeing it as insufficient in the face of an economic recovery that appears to be sputtering out. U.S. stock prices were under considerable pressure Wednesday, reflecting the market’s anxiety over the economic outlook.

The New York Fed said after the FOMC meeting that while the central bank would buy Treasurys across the yield curve, it would concentrate on issues maturing between 2 and 10 years, and that it would avoid issues in which it already had large holdings.

The $18 billion the New York Fed will put into Treasurys will come from the proceeds of mortgage securities now on the New York Fed’s books. It will announce its next round of purchases on Sept. 13.