Washington, Dec 10 (DPA) The US is probing the possibility that hackers have been carrying out attacks on the websites of financial companies that have halted payments to WikiLeaks, a top US law enforcement official has said.

"We are aware of the incidents," Attorney General Eric Holder said Thursday. "We're looking into them."

Holder spoke to reporters after a meeting with EU security and justice officials in Washington.

Hackers proclaiming to back WikiLeaks' ongoing release of 250,000 secret US diplomatic cables have declared online that they are trying to obstruct access to Mastercard and Visa websites. The credit card companies have been used by individuals seeking to donate money to WikiLeaks.

The US government has vehemently condemned the WikiLeaks release. Holder would not say whether the government has been urging companies to halt money transfers to WikiLeaks.

The two credit card companies announced earlier this week they would no longer accept payments to WikiLeaks. The online service PayPal did the same. The hackers, calling themselves "Anonymous", have vowed to go after PayPal next as part of "Operation Payback".