James Clapper, the US Director of National Intelligence, made the comments in testimony to the US Senate.
From Canada and Britain, France and South Korea, there have been growing reports of online attacks on computer networks around the world.
The finger of suspicion is often pointed at China.
In evidence to the US Senate’s Armed Services Committee, Mr Clapper said last year saw a “dramatic increase” in malicious cyber-activity targeting US computers and networks.
He did not mention who might be behind the surge in attempts to steal information from government ministries, major companies and others.
But he added that China had made “a substantial investment” in cyber-warfare and intelligence gathering, saying it had a “very large organisation devoted to it and they’re pretty aggressive”.
“This is just another way in which they glean information about us and collect on us for technology purposes, so it’s a very formidable concern,” he said.
He cited the incident of 8 April 2010, when state-owned China Telecom advertised erroneous network routes that instructed “massive volumes” of internet traffic to go through Chinese servers for 17 minutes.
“This incident affected traffic to and from US government and military sites, including sites for the Senate, the Army, the Navy, the Marine corps, the air force, and the office of the Secretary of Defense, as well as a number of Fortune 500 firms,” he said.
China Telecom denied that it had hijacked US internet traffic.
China routinely denies such charges, and has in the past accused the US of using “online warfare” to foment unrest in nations like Iran.
Beijing often adds it is a victim rather a perpetrator of cyber-attacks.
It criticised as “irresponsible” an allegation from a South Korean politician earlier this week blaming Chinese hackers for intrusion into Seoul’s military files.
“I want to emphasise that Chinese law prohibits any cyber-attacks including hacking of any form and fights against these types of crimes in accordance with the law,” a Chinese foreign ministry spokeswoman Jiang Yu told reporters.