Suspected Russian hackers spied on U.S. Treasury emails - sources

The U.S. Treasury building is seen in Washington, September 29, 2008. REUTERS/Jim Bourg (UNITED STATES)

WASHINGTON (Reuters) - Hackers believed to be working for Russia have been monitoring internal email traffic at the U.S. Treasury and Commerce departments, according to people familiar with the matter, adding they feared the hacks uncovered so far may be the tip of the iceberg.

The hack is so serious it led to a National Security Council meeting at the White House on Saturday, said one of the people familiar with the matter.