The rejection follows reports quoting mostly unnamed officials pointing the finger at the Chinese military over cyber-espionage attacks on government computer networks in the US, UK and Germany.
On Tuesday Pentagon officials admitted that hackers had gained access to an unclassified email system in the office of the US defence secretary.
But a spokesman declined to comment on reports that a division of China's People's Liberation Army (PLA) was responsible for the latest hack.
"It is often very difficult to pinpoint the true origin of a particular intrusion," the official said.
However a report in the Financial Times, citing former and serving US officials, said the PLA was behind the hack, described as the most successful cyber attack yet on Pentagon networks.
|The Pentagon says China's army emphasises |
hacking as an offensive weapon [AP]
The FT cited one source as saying there was a "very high level of confidence ... trending towards total certainty" that the PLA was to blame.
The US and China are both widely believed to conduct cyber-espionage against each other but the issue rarely goes public.
On Wednesday Britain's Guardian newspaper, citing unnamed government officials, reported that PLA-linked hackers had also been attacking the computer networks of British government departments.
Among those targeted were networks in the foreign ministry and the House of Commons, the lower house of the British parliament, which was forced to shut down part of its network following a hack attack last year.
The reports come a week after China deflected claims in German magazine Der Spiegel's reporting that hackers backed by the Chinese military had gained access to the German foreign ministry and the offices of Angela Merkel, the German chancellor.
Merkel was reported to have raised the issue with Chinese leaders during a visit to Beijing last month.
'Never a threat'
According to the Pentagon, the email security breach occurred a few months ago and caused the system to be taken down for three weeks, but was "never any threat to the classified systems".
"We've got great relations with China, from a diplomatic perspective"
It added that hackers attempt to infiltrate the Pentagon's system hundreds of times a day.
Earlier this year a Pentagon study said that China's military emphasises hacking as an offensive weapon, citing Chinese exercises in 2005 that included hacking "primarily in first strikes against enemy networks".
The reports on the alleged hacking come as the US president, George Bush, is scheduled to hold talks with his Chinese counterpart at the Asia-Pacific Economic Cooperation (Apec) summit in Sydney this week.
Asked to comment on the reports on Wednesday, Bush largely side-stepped the issue, saying "we understand that we're vulnerable in some systems".
"We've got great relations with China, from a diplomatic perspective," he added.