[Updated at 5:15 p.m.] ﻿The corporate websites of Visa and MasterCard

were inaccessible at times Wednesday due to an apparent cyberattack by purported Wikileaks backers.

Messages posted on Twitter indicated the attacks may be in response to recent moves by Visa and MasterCard against WikiLeaks, the website that recently released thousands of secret U.S. State Department documents.

"The issue appears to be the result of a concentrated effort to flood our corporate website with traffic and slow access," said MasterCard spokesman James Issokson, in a prepared statement. "We are working to restore normal service levels."

MasterCard and Visa Europe, a division of Visa, had said they were working to stop the acceptance of their cards on WikiLeaks.