"It's effective and I'm not concerned about the amount of uptake on that ... or whether it would increase further," Williams told reporters. "It would just be a sign that it's working as planned."

Usage of the program, known as the reverse repo facility, surged further after the Fed lifted the rate it pays firms to 0.05% from zero at last week's policy meeting, a move Williams said was aimed at helping to keep the Fed's key benchmark rate within the target range of zero to 0.25%. Firms parked a record $765 billion overnight with the Fed on Monday.

(Reporting by Jonnelle Marte; editing by Grant McCool)