China's People's Liberation Army (PLA) is contining to develop viruses to attack computer systems, warned the US Department of Defense (DoD).

"The PLA has established information warfare units to develop viruses to attack enemy computer systems and networks," the annual DoD report on China's military warned. At the same, Chinese armed forces are developing ways to protect its own systems from an enemy attack, it said, echoing similar warnings made in previous years.

These capabilities are part of China's ongoing military modernisation efforts, which have seen the country add dozens of high-tech fighters and ballistic missiles to its arsenal. China isn't alone in building the capability to attack an enemy's computer systems. The US and other countries have developed similar abilities.

The PLA's virus-writing efforts have been underway for years, reflecting the importance that China apparently attaches to information warfare. As early as 2000, the DOD warned, "China has the capability to penetrate poorly protected US computer systems and potentially could use CNA [computer network attacks] to attack specific US civilian and military infrastructures."

In recent years, the PLA has begun training more seriously for computer attacks, including them as part of larger military exercises in 2005.

The main focus of China's military modernisation efforts are Taiwan, an island nation that China views as a renegade province. China has long threatened to attack Taiwan if the island formally declares independence, and the expansion of China's military capabilities are largely geared towards a possible attack against Taiwan.

"A limited military campaign could include computer network attacks against Taiwan’s political, military, and economic infrastructure to undermine the Taiwan population’s confidence in its leadership," the report said.

But the US, which would likely intervene in a Chinese attack on Taiwan, is also a potential target, it said.