"The threats and the vulnerabilities within the cyber domain are serious and significant," John Brennan told NBC, adding a cybersecurity board had been set up at the White House.

"It's a very complex and complicated challenge, but we are working very closely with the private sector."

Last month a row over Internet freedom erupted between China and the United States as Beijing denied any state involvement in cyberattacks on Internet search engine Google.

President Barack Obama said he was "troubled" by Google's statements it had been attacked by China-based hackers, and demanded official answers.

The US Internet giant has threatened to abandon its Chinese search engine, and perhaps end all operations in the country over the cyberattacks. PTI