The Chinese "clearly have potential to put some of our capabilities at risk," Gates told the BBC before landing. "We have to pay attention to them; we have to respond appropriately with our own programs."
Gates is to meet President Hu Jintao and other officials days after photos surfaced of China's first stealth aircraft, invisible to radar. China has not officially commented on the photos of a runway test last week but has not denied them either.
The U.S. Air Force currently has the world's only operating stealth fighter, the Lockheed Martin F-22 Raptor.
The United States has been watching as China boosts its military capacity, particularly by developing a "carrier-killer" missile, a land-based system that could sink an aircraft carrier from up to 1,800 miles away, the report said. U.S. battle groups, including carriers, are stationed in the South China Sea.
In August, the Pentagon reported that China had been expanding its nuclear and submarine forces along with land-based missiles.
Japan last month officially called China its primary military concern.