The Fed Chairwoman Janet Yellen said in a speech on Monday that further interest rate hikes were appropriate as the U.S. labour market continued to strengthen and inflation was picking up toward 2% target.

"I expect the U.S. economy will continue to improve and why I expect that further gradual increases in the federal funds rate will probably be appropriate to best promote the FOMC's goals of maximum employment and price stability," she said.

Yellen pointed out that there were uncertainties to the outlook for the economic growth and to the path of the federal funds rate. She noted that Britain's exit from the European Union (EU) could have a negative impact on the U.S. economy.