Federal Reserve Bank of St. Louis President James Bullard said it may be difficult to tie the central bank’s $85 billion monthly bond purchases to numerical levels of unemployment and inflation.

After settling a debate over how long to hold interest rates near zero, Fed officials are debating when to halt their purchases of Treasuries and mortgage-backed securities. At its December meeting, the Federal Open Market Committee agreed to hold the target interest rate near zero so long as unemployment remains above 6.5 percent and inflation stays below 2.5 percent.