Ms Napolitano said the US was capable of launching its own cyber attacks
The issue of cyber security is of "great concern" to the US, the nation's homeland security secretary has said.
Janet Napolitano told the BBC that protecting against virtual attacks was something the US was "moving forward on with great alacrity".
Speaking on a visit to the UK, she said the US also had "a number of capabilities" to launch such attacks.
Ms Napolitano's comments follow the announcement last month of a new cyber security office in the White House.
Ms Napolitano said cyber attacks were an "area where critical infrastructure can be affected and where economic damage can be done".
She said the US government took all aspects of cyber crime very seriously and that the US defence department had formed "an entire cyber command" to handle online threats.
When asked whether the US had been the instigator of cyber attacks, Ms Napolitano told the BBC: "I think there are a number of capabilities with which to do that, yes."
Last month, US President Barack Obama said the cyber threat was "one of the most serious economic and national security challenges we face as a nation".
He said the country's computer networks were "a national security priority" and announced a multi-billion plan to protect them against attack, including the creation of a new cyber security department.
In 2007 alone, the Pentagon reported nearly 44,000 incidents of what it called malicious cyber activity carried out by foreign militaries, intelligence agencies and individual hackers.
The UK has also launched a cyber defence programme and indicated that it, too, has cyber warfare capabilities.