U.S. considers 'proportional' response to Sony hacking attack

An entrance gate to Sony Pictures Entertainment at the Sony Pictures lot is pictured in Culver City, California in this April 14, 2013 file photo. REUTERS/Fred Prouser/Files

WASHINGTON (Reuters) - The United States said on Thursday a cyber attack on Sony Pictures was a serious national security matter and the Obama administration was considering a proportional response, although the White House stopped short of blaming North Korea.

U.S. government sources said on Wednesday that investigators had determined the attack was "state sponsored" and that North Korea was the government involved.