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 plane made a 15-minute test flight before landing.
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.
It was revealed last week that China appeared to have completed a prototype of its first stealth fighter, highlighting Beijing`s military modernisation drive.
China`s defence ministry was not immediately available for comment.
"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.
"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."
However, he stopped short of confirming the existence of the plane or apparent test flight.
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 openess 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.
"There are two reasons they are announcing this, one is for deterrence and the other is transparency," Chang said.
It is very rare for Chinese media to publicise news on military or other sensitive matters without official blessing.
"They want to show their power to the United States, to tell the American people, especially Gates, `don`t look down on us`." (*)
Editor: Kunto Wibisono
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