WASHINGTON (AP) — Federal Reserve Chairman Jerome Powell says that the pace of improvement in the economy has moderated in recent months with future prospects remaining “extraordinarily uncertain.”

In remarks released by the Fed on Monday, Powell said that the increase in new COVID-19 cases both in the United States and abroad was “concerning and could prove challenging for the next few months. A full economic recovery is unlikely until people are confident that it is safe to reengage in a broad range of activities.”

Powell said while progress on developing vaccines had been “very positive,” significant challenges remained regarding the timing, production, and distribution of the vaccines, and it remained difficult to assess the economic implications of this process with any degree of confidence.

Powell’s remarks were prepared for a joint appearance he will make on Tuesday with Treasury Secretary Steven Mnuchin before the Senate Banking Committee. The hearing is part of the panel’s oversight responsibilities required under the multi-trillion economic support legislation Congress passed in the spring.