The U.S. military's biggest worry, he said, is what are known as China's "anti-access and area-denial" weapons, including submarines and the anti-ship missile, designed to prevent the U.S. from operating without fear in the Western Pacific.
Those weapons go beyond China's defensive needs and "threaten our primary means of projecting power: our bases, our sea and air assets, and the networks that support them," Gregson said.
Vice Adm. Dorsett said it was unclear when the aircraft would be operational.
"They have been able to invest in a military buildup, and a stealth fighter is just one aspect of that," he said. "The fact they are making progress in that should not be a surprise."
Dorsett said he was more troubled by China's advances in space weapons and cyberwarfare capabilities. In 2007, China demonstrated that it could shoot a satellite out of low Earth orbit. And for years, corporate and government computer systems in the U.S. and elsewhere, including those of American defense contractors, have been hit by cyberattacks traced to China, though a link to the Chinese military hasn't been publicly established.
Some experts believe Chinese military hackers already have the ability to take down U.S. power grids and disrupt the financial system.
China is also developing and fielding "large numbers of advanced medium-range ballistic and cruise missiles, new attack submarines equipped with advanced weapons [and] increasingly capable long-range air defense systems," says the U.S. military's 2010 Quadrennial Defense Review.
"It's China's goal to have a globally deployable military by the 2020s," said Fisher, of the Virginia think tank. "We have to understand that what the Russians teach them they are absorbing well. They are becoming military technology innovators, not just copiers."
China has nuclear weapons and a modern air force, but it doesn't have an aircraft carrier or bases abroad; its main military focus has been Taiwan, the island allied with the U.S. that China considers a province, despite it being ruled separately since the end of a civil war in 1949. But China is building as many as five aircraft carriers, analysts say, and is increasingly turning its focus to projecting power beyond the Taiwan Strait.
China is the world's second-largest military spender after the U.S., though the gap is large. China put its 2010 defense budget at nearly $80 billion. The sum is less than a fifth of the U.S. level of about $530 billion, which doesn't include costs in Iraq and Afghanistan, but the U.S. believes the amount spent by China is higher.