The Financial Times, quoting unnamed officials on Monday, attributed the attacks to China's People's Liberation Army. Defense Department spokesman Maj. Patrick Ryder said he could not confirm the report.

China is believed to aggressively probe U.S. networks to gain intelligence. In June, Defense Secretary Robert Gates confirmed during a briefing that 1,500 e-mail users were taken offline after the intrusion.

The department said pinpointing the origin of the attacks is difficult since hackers often use other computers they've hacked to launch attacks.

The department's Global Information Grid, which encompasses warfighting and civilian systems, is constantly scanned for weaknesses. Those hackers are looking for exploits in commercial software and operating systems, the department said.

"We know that a number of nations and groups are actively developing these capabilities," according to a statement. "We have seen attempts by a variety of state and non-state sponsored organizations to gain unauthorized access to, or otherwise degrade, [Defense Department] information systems."