NEW YORK (CNNMoney.com) -- 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 maybe 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.
Issokson said the hack attack did not affect the use of credit cards or financial security. The networks used to run credit card transactions operate independently from corporate websites.
By Wednesday evening, MasterCard said it had "made significant progress in restoring full-service to its corporate website."
Visa's site -- which appeared to attract attacks later in the day -- remained inaccessible.
"Visa's processing network, which handles cardholder transactions, is functioning normally and cardholders can continue to use their cards as they routinely would. Account data is not at risk," Visa spokesman Ted Carr said in a statement.
Carr added that Visa.com is experiencing "higher than normal" traffic and that the company is taking steps to restore the site to full operations within the next few hours.
The action against Visa comes after Visa Europe, a division of Visa, stopped accepting payments for WikiLeaks.
"Visa Europe has taken action to temporarily suspend Visa payment acceptance on WikiLeaks' website pending investigation into whether it contravenes Visa operating rules, including compliance with local laws in the markets where we operate," said Visa, in a prepared statement.
Issokson wouldn't comment on allegations of who was behind the attack. But on Tuesday, MasterCard said it was "working to suspend the acceptance of MasterCard cards on WikiLeaks."
On Twitter, a post from a handle called @Anon_Operation took responsibility for the attacks, tweeting about its so-called Operation Payback: "We are glad to tell you that [Mastercard.com] is down and it's confirmed. Operation: Payback (is a bitch!)"
Just before the apparent attack on Visa, the same alias posted: "TARGET: WWW.VISA.COM :: FIRE FIRE FIRE!!! WEAPONS."
There was no immediate confirmation that whoever was behind the handle was responsible for the attack.
The website for PayPal was also the subject of an "attempted DDoS [denial-of-service] attack" in recent days, though the site remained operational, according to PayPal spokeswoman Charlotte Hill.
"These attacks have at times slowed the website itself down, but have not significantly impacted payments," she said.
Hill confirmed that this happened after PayPal denied service to WikiLeaks.
PostFinance, a Swiss bank, also had problems with its website on Tuesday, when it announced that the site was "overloaded owing to a multitude of online enquiries."
This happened the day after PostFinance announced that it closed the account of WikiLeaks founder Julian Assange for having "provided false information regarding his place of residence when opening the account."
The PostFinance website was back up Wednesday.
Credit.com reported, on its blog, that credit card companies and financial firms were making it difficult for supporters to funnel cash to WikiLeaks.
"With such major players cutting off its access to funds, the financial pressure on WikiLeaks appears to be mounting," wrote Credit.com on its blog. "The site can accept donations through DataCell, a Swiss credit card company, and through bank transfers to accounts in Germany and Iceland, though the decision by Visa and MasterCard to block transfers may make it difficult for those channels to remain open."
DataCell reported Wednesday that MasterCard and Visa payments were being rejected in its "donation system" through the WikiLeaks website. DataCell said it was taking "immediate legal actions to make donations possible again."
"The suspension of payments towards WikiLeaks is a violation of the agreements with their customers," said DataCell, in its news release. "This does clearly create massive financial losses to WikiLeaks which seems to be the only purpose of this suspension."