The Fed Chair Janet Yellen testified before the Senate Banking Committee on Wednesday. She said that she expects the U.S. economy to strengthen in the rest of the year. Yellen pointed out that the Fed could raise its interest rate "at some point this year".

"Looking forward, prospects are favourable for further improvement in the U.S. labour market and the economy more broadly," she said.

The Fed chairwoman pointed out that the U.S. dollar could expand faster than expected in the second half of the year due to lower gasoline prices and as a negative impact on the U.S. exports due to a stronger U.S. dollar begins to fade.

She noted that there is "still some slack" in the U.S. labour market.

Yellen also said that the situation in Greece and China could "pose some risk" to the U.S. economy.