Google's recently reported cyberattacks are a "wake-up call" about the vulnerabilities of computer networks to attacks that could cripple the US economy, the top US intelligence official warned Tuesday.

"We cannot be certain that our cyberspace infrastructure will remain available and reliable during a time of crisis," US Director of National Intelligence Dennis Blair told key lawmakers.

Blair said vital telecommunications and information systems faced threats from, "those who would steal, corrupt, harm or destroy the public and private assets vital to our national interests."

He pointed to Google's announcement that following cyberattacks on the Gmail accounts of Chinese human rights activists it was no longer willing to censor Web search results in China even if that means it has to leave the country.

"The recent intrusions reported by Google are a stark reminder of the importance of these cyber assets, and a wake-up call to those who have not taken this problem seriously," Blair told the Senate Intelligence Committee.

"China's aggressive cyber activities" are among activities by Beijing that "pose challenges" to its neighbors and beyond, he told lawmakers.

Blair said "terrorist networks, organized criminal groups, individuals, and other cyber actors" have targeted US systems "for intelligence collection, intellectual property theft, or disruption."

"Malicious cyber activity is occurring on an unprecedented scale with extraordinary sophistication," he warned, stressing that "sensitive information is stolen daily from both government and private-sector networks."

In addition to rival powers, crime syndicates, and extremist groups, some of the threat comes from unknown sources.

"We often find persistent, unauthorized, and at times, unattributable presences on exploited networks, the hallmark of an unknown adversary intending to do far more than merely demonstrate skill or mock a vulnerability," he said.

Blair warned criminals have been targeting the latest generation of cellular telephones "whose increasing power and use in financial transactions makes them potentially lucrative targets."

He said he was pursuing efforts to toughen US cyberdefenses and that "new cybersecurity approaches must continually be developed, tested, and implemented to respond to new threat technologies and strategies."