The Fed Chairwoman Janet Yellen wrote in a letter to consumer advocate Ralph Nader on late Monday evening that the Fed starts raising its interest rate if the labour market will continue to strengthen and the inflation will toward the Fed's 2% target.

"We all hope and expect that the economy will continue to expand, that the jobs market will continue to make progress, and that inflation will move toward our two percent price stability objective," she wrote.

Yellen added that interest rate hikes will be gradual.

The Fed chairwoman defended low interest rates, saying that it helped to stimulate the economy.

Nader posted online an open letter to Yellen on October 30. He asked why the Fed was keeping interest rates at low levels.