WASHINGTON -- The United States faces a mounting danger from cyber attacks on its infrastructure while digital espionage threatens to undercut the military's technological edge, the intelligence chief said Tuesday.

Citing "increasing risk to U.S. critical infrastructure," National Intelligence Director James Clapper said in an annual report to Congress that "unsophisticated" attacks could penetrate poorly protected computer networks for power grids or similar systems.

The threat of a large-scale digital assault that could cripple a regional power network was genuine but remained a "remote" possibility, the report said.

"We judge that there is a remote chance of a major cyber attack against U.S. critical infrastructure systems during the next two years that would result in long-term, wide-scale disruption of services, such as a regional power outage," it said.

The report placed more importance on cyber threats than previous years, with more words on the problem than on Islamist militants in Afghanistan.

Countries with advanced cyber capabilities, such as Russia and China, were unlikely to launch a massive digital assault on the United States unless there was a military conflict or grave crisis that put their national interests at risk, according to the report.

But "there is a risk that unsophisticated attacks would have significant outcomes due to unexpected system configurations and mistakes, or that vulnerability at one node might spill over and contaminate other parts of a networked system," it said.

Copyright Agence France-Presse, 2013