National Security Advisor John Bolton said the country's "first fully articulated cyber strategy in 15 years" was now in effect.

The new more aggressive posture follows a decision by President Donald Trump to revoke rules established by his predecessor Barack Obama to require high-level authority for any big military cyber operations.

"Our hands are not tied as they were in the Obama administration," Bolton said.

"For any nation that's taking cyber activity against the United States, they should expect... we will respond offensively, as well as defensively," Bolton said.

"Not every response to a cyber attack would be in the cyber world," he added.

Bolton referred to China, Iran, North Korea and Russia as major sources of threats, saying "Americans and our allies are under attack every day in cyberspace."

Hostile intrusions target everything from US infrastructure to government bureaucracies, businesses and elections, according to Bolton.

Among the most notorious incidents is the hacking, allegedly by Russian military agents, of Democrat figures' email accounts just before the 2016 elections, in which Republican Trump took a surprise win against the heavily favored Hillary Clinton.