US intelligence agencies have placed cyber attacks from foreign governments and criminals at the top of their list of threats to the country.
Director of National Intelligence James Clapper said online assaults would increasingly undermine US economic competitiveness and national security.
A report issued by his office said Russia's military was setting up a cyber command to carry out attacks.
The report also describes China, Iran and North Korea as leading threats.
Mr Clapper said he no longer believed the US faced cyber "Armageddon" in testimony to a congressional committee on Thursday.
He said the idea that major infrastructure such as financial networks or power grids could be disabled by hackers now looked less probable.
However he warned: ""We foresee an ongoing series of low-to-moderate level cyber attacks from a variety of sources over time, which will impose cumulative costs on US economic competitiveness and national security."
Mr Clapper said Russia posed the greatest cyber risk to US interests and added that the threat from Russian government was more "severe" than previously realised.
He added that profit-minded criminals and ideology-driven hackers were also increasingly active.
Over the past year there have been a series of high-profile cyber attacks against US targets.
North Korea was accused of being behind the theft of a huge data cache from Sony Pictures back in November.
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