The US Justice Department is looking into cyber attacks on opponents of WikiLeaks and companies that have stopped doing business with it, Attorney General Eric Holder has said.

Holder made the comment at a news conference yesterday after a meeting on cybersecurity, counterterror and data protection with European Union law enforcement partners.

Hackers launched attacks on MasterCard, Visa, Swedish prosecutors, a Swiss bank and others who have acted against the WikiLeaks site and its jailed founder, Julian Assange.

"We are aware of the incidents ... and we are looking into them," Holder said.

The attorney general said he is hopeful that the people responsible for the WikiLeaks disclosures of classified information will be brought to justice.

The participants at the law enforcement meeting at the Justice Department discussed privately their misgivings about WikiLeaks' releases of the classified material.

The European Union and the United States are coordinating to protect vital cybernetworks from attacks, said Homeland Security Secretary Janet Napolitano, the other Cabinet official at the news conference.

Napolitano also said the United States and the EU are extensively collaborating on cargo security efforts. In October, authorities thwarted a terrorist plot to ship explosive devices on aircraft travelling through Europe to the US.