Foreign profits held overseas by U.S. corporations to avoid taxes at home nearly doubled from 2008 to 2013 to top $2.1 trillion, a private research firm's said in a report, prompting a call for reform by the Senate's top tax law writer.

"The new numbers . . . certainly highlight what is one of the key challenges for tax reform. I do think there need to be some reforms in this area," Senate Finance Committee Chairman Ron Wyden told reporters Tuesday on Capitol Hill.

Under U.S. law, corporations do not have to pay income tax on most of their overseas profits until they are brought into the United States. These earnings can be held offshore for years if they are classified as indefinitely invested abroad.