China's first stealth fighter jet has made its maiden flight, according to photos published by state media on Tuesday, just as US Defense Secretary Robert Gates paid a key visit to Beijing.

The photos, published on the websites of the official Xinhua news agency and Global Times newspaper and said to have been taken by aviation enthusiasts, showed the alleged J-20 fighter flying over the southwestern city of Chengdu.

According to the reports, which cited witnesses, the next-generation war jet -- the existence of which highlights China's military modernisation drive -- made a 15-minute test flight before landing.

But one senior US official traveling with Gates said the Chinese military had carried out the test flight without first informing China's President Hu Jintao and the country's civilian leadership.

"It was clear that none of the civilians in the room had been informed (of the test flight)," said the official, who spoke on condition of anonymity, describing a meeting between Hu and Gates.

When the Pentagon chief asked Hu directly about the test flight during their meeting at the Great Hall of the People, the Chinese president and his aides were clearly caught off-guard, the senior official said.

Hu "said the tests had nothing to do with my visit," and had been previously scheduled, said Gates, who added he accepted the explanation.

Gates is in Beijing on a fence-mending trip one year after China suspended military ties with the United States over Washington's sale of billions of dollars in arms to Taiwan, but the flight threatened to overshadow that effort.

It was revealed only last week that China had completed a prototype of the J-20, which experts say will eventually rival the US Air Force's F-22, the world's only fully operational next-generation stealth fighter jet.

Asked about his view of the People's Liberation Army (PLA) in light of the test flight, Gates said: "I've had concerns about this over time.

"It's one of the reasons why I attach importance to a dialogue" on security issues between both countries, he added.

China's massive annual military spending has aroused concern.

US military officials and strategists see Beijing as a potential threat to Washington's once unrivalled dominance of the Pacific.

But Beijing has repeatedly insisted its military growth does not pose any threat.

"China pursues a defence policy that is defensive in nature and poses no threat to any other nation," foreign ministry spokesman Hong Lei said when asked about the test flight, though he stopped short of confirming it.

"With the development of science and technology and according to the needs of national defence-building, it is natural that countries update their weapons and equipment."

Andrei Chang, the head of the Kanwa Information Centre, which monitors China's military development, said the flight was a "factory test" and not a comprehensive test by the Chinese air force.

"The fighter still needs to undergo more serious tests," Chang told AFP.

More importantly, China's openness on the plane represented a new level of military transparency for the nation which is well-known for maintaining strict secrecy in its political and military affairs, he said.

It is very rare for Chinese media to publicise news on military or other sensitive matters without official blessing.

"There are two reasons they are announcing this -- one is for deterrence and the other is transparency," Chang said.

"They want to show their power to the United States, to tell the American people, especially Gates, 'don't look down on us'."