Bryan Whitman said it was US policy to encourage China to emerge as a responsible international partner.
"However, there is also a lack of transparency and some uncertainty surrounding China's future path. Therefore, we and others have to naturally hedge against the unknown," he told reporters.
His comments came as Hu Jintao, the Chinese president, was at the White House meeting with George Bush on a range of trade and security issues, and to assure US leaders they had nothing to fear from China's rising might.
Hu's visit comes against a backdrop of increasing US concern about China's intentions as it pursues a major military buildup that the Pentagon believes will threaten the military balance in region.
The United States also has been modernising and re-orienting its military forces in recent years, shifting its weight from Europe to the Asia-Pacific region and south Asia.
US has revamped its military alliance with Japan, and moved to strengthen military ties with India and countries in southeast and central Asia.
"There is also a lack of transparency and some uncertainty surrounding China's future path. Therefore, we and others have to naturally hedge against [that]"
Bryan Whitman, Pentagon spokesman
Guam is being transformed into a hub for long-range bombers, intelligence and surveillance aircraft and logistics support. The military plans to move 8000 Marines to Guam from Okinawa, Japan by 2012.
The US Navy, meanwhile, is building up its presence in the region.
It is adding a sixth aircraft carrier to the Pacific Fleet and has decided to home port 52 attack submarines - 60% of its fleet - in the Pacific theater by 2010.
The air force is investing billions of dollars to acquire costly F-22 fighter aircraft capable of cruising at supersonic speeds and to develop a new long range bomber, all with an eye on China.
US Defence policy
Pentagon and US military officials in the past have insisted that the US military realignment was not directed at any specific country, or aimed at containing China.
Rumsfeld has spoken out against
a potential Chinese military threat
But Whitman's acknowledgment that the changes were a "hedge" against China indicates Washington is opting for a more candid approach in spelling out the consequences of Beijing's military build up.
The change in tone began last June when Donald Rumsfeld, the US defence secretary warned at an international security conference in Singapore that China was spending much more on its military than officially acknowledged.
A major Pentagon strategy review made public in February singled out China as having "the greatest potential to compete militarily with the United States and field disruptive military technologies that over time offset traditional US military advantages absent US counter-strategies."