U.S. Federal Reserve Chairman Jerome Powell on Wednesday said the COVID-19 crisis raises "long-term concerns," warning that a delayed retreatment and weak recovery could lead to an extended period of low productivity growth and stagnant incomes.

"The scope and speed of this downturn are without modern precedent, significantly worse than any recession since World War II," Powell said in prepared remarks webcast by Washington-based think tank Peterson Institute for International Economics (PIIE).

Powell noted the record shows that deeper and longer recessions can leave behind "lasting damage" to the productive capacity of the economy.

Noting that the result could be an extended period of low productivity growth and stagnant incomes, the Fed chairman said more policy measures might be needed to support the recovery and avoid delayed treatment.