But he said many questions about Beijing's military remain unanswered, including how China plans to use its rapidly growing and modernizing force.

Gen. Paul V. Hester, the Pacific Air Forces commander, told reporters during a conference call that he visited two military bases during his late July trip, including one called Jining never before visited by an American commander.

While there, he saw the Su-27, a Russian-made supersonic jet fighter, and met the aircraft's flight and maintenance crews.

Hester said this showed progress compared with previous U.S. military visits, noting Gen. Peter Pace, the chairman of the Joint Chiefs of Staff, was only able to see an Su-27 that was kept at a non-operational Chinese base.

Hester's staff specifically asked that he be allowed to see advanced planes in their normal operating environment, and China accommodated the request, he said.

Hester also met China's air force chief in Beijing and visited the Chinese air force's command college where he took questions from a group of some 30 officers.

The four-star general said he was pleased with the access afforded him during his trip.

"The question is, is it progressing?" Hester asked. "Absolutely."

The U.S. military relaunched efforts two years ago to expand exchanges with China's military to help the two powers better understand each other and to avoid unintended conflict. Such exchanges largely broke off in 2001 after a U.S. spy plane collided with a Chinese fighter jet.

U.S. critics of the policy say the U.S. is opening itself up to extensive scrutiny by China without securing the same level of access from the Asian nation.

Hester said there were still some "unknowns" about China's military, particularly how Beijing plans to use its forces. The people Hester spoke with didn't explain their thinking on this issue, he said.

"There's certainly not much solid answers to that question of 'What is your vision for your military and where will it lead you?' " Hester said.

The Pentagon has been watching closely how China had been buying new airplanes, submarines and missiles in recent years to modernize its military.

In an annual report prepared for Congress in May, the Pentagon said China was modernizing its military in ways that give it options for launching surprise attacks, potentially far from its borders.