The United States and a dozen other countries are staging a huge cyber exercise to test the preparedness of their computer networks against a simulated attack, Homeland Security Secretary Janet Napolitano has announced.

The exercise, dubbed "Cyber Storm III", simulates a "large-scale cyberattack on critical infrastructure" and involves thousands of participants from seven US government departments, 11 US states, 60 private sector companies and 12 international partners.

"Securing America's cyber infrastructure requires close coordination with our federal, state, international and private sector partners," Napolitano said in a statement late yesterday.

"Exercises like Cyber Storm III allow us build upon the significant progress we've made in responding to evolving cyber threats," she said.

The biennial exercise is being organised by the Department of Homeland Security and is the first test of the new National Cybersecurity and Communications Integration Center based outside Washington in Arlington.

The NCCIC booted up in October 2009 to serve as the coordinating center for US cybersecurity operations and houses US government computer experts and their private sector counterparts under one roof.

The international partners taking part in the exercise are from Australia, Britain, Canada, France, Germany, Hungary, Japan, Italy, the Netherlands, New Zealand, Sweden and Switzerland.