Schaffer said the government is tracking more and more cyber attacks.
The U.S. government is seeing a rise in cyber attacks aimed at taking over control systems that operate critical infrastructure, such as industrial facilities and pipelines, a senior Homeland Security Department official said Tuesday.
"It's certainly a trend," Greg Schaffer, assistant secretary for the DHS Office of Cybersecurity and Communications, told reporters.
Schaffer said the government is tracking more and more cyber attacks that have a greater level of sophistication and are tailored to target specific types of industrial infrastructure, such as power grids.
"These are the kinds of things that do give us pause," he said during an event hosted by the Defense Writers Group.
"It is widely recognized that the cyber ecosystem we have today favors the offense and not the defense," Schaffer added. "It is simply too hard to secure the systems."
The so-called Stuxnet worm, which affects Windows-operating systems responsible for running supervisory control and data acquisition systems, is one such example, he said.
There is no indication that the threat from Stuxnet has subsided, and the potential remains for some U.S. infrastructure to be attacked by it, he said.
But Schaffer declined to comment on the origin of the worm - a question that has stirred considerable speculation but no answers. He also dodged several other hot-button issues, including what the government is doing in response to the disclosure by WikiLeaks of confidential U.S. diplomatic cables.
U.S. officials suspect an Army intelligence analyst stole the documents using the Pentagon's Internet for secret information, commonly referred to as SIPRNet. Schaffer noted that the Pentagon is responsible for the protection of its networks.
But he added that defending information technology networks requires a balance between having protective measures but not impeding the ability of government workers to carry out their duties. "Security is an exercise in risk management," he said.
The Homeland Security Department and National Security Agency recently signed a memorandum of agreement to cooperate on cybersecurity efforts and share personnel.
"This structure is designed to put the full weight of our combined capabilities and expertise behind every action taken to protect our vital cyber networks, without altering the authorities or oversight of our separate but complementary missions," Homeland Security Secretary Janet Napolitano and Defense Secretary Robert Gates said in a joint statement on Oct. 13.
Schaffer described the agreement as solidifying a relationship in which DHS and NSA work together on daily issues.
He said there has not been a significant cyber event since the agreement was signed.