There is no evidence that data was taken in the incident, the official said, adding that the attack was identified early and did not spread.

The attack was described as "spear phishing," the term for an attempted penetration using fake emails from a trusted sender, which the official said was "not infrequent."

On Sunday, a report from a news website "Freebeacon," that describes itself as an alternative to "the professional left," said that Chinese hackers had breached a White House military system.

The White House would not identify the group responsible for the attack, or give specifics about its timing and target.

China has the world's largest Internet user base, at 485 million users, and is believed to be responsible for a number of hacking attacks aimed at the U.S. government and companies.

"In this instance the attack was identified, the system was isolated, and there is no indication whatsoever" that any data was extracted, the official said.

"Moreover, there was never any impact or attempted breach of any classified system."

The Obama administration is preparing to issue an executive order that would direct federal agencies to develop new guidelines to shield computer networks from cyber attacks. The White House undertook the new rules after Congress failed earlier this year to pass a comprehensive cybersecurity bill.

(Reporting By Margaret Chadbourn; Editing by Sandra Maler)

• A Permanent Economic Emergency