A "huge globular luminous object" was spotted in the Shanghai sky on Saturday evening by aircrews who reported it to air traffic control, the East China Air Traffic Control Bureau confirmed to media on Tuesday.
"Several flights" had reported the same sight the same night, an unnamed bureau official told the Shanghai Daily.
The bright object was "far" from the planes, according to the bureau, and appeared "normal" on the radar, the Shanghai Oriental Morning Post reported. Flight control personnel therefore did not request pilots avoid it.
A microblogger claiming to be the pilot of Flight CZ6554 from Shanghai to Changchun wrote he had seen a huge bright spherical object at 10,700 meters above Shanghai about 9 pm on Saturday.
The object grew larger and appeared as a regular geometric sphere hundreds of times larger than the Moon, he said.
"Its diameter reached as long as 50 nautical miles," the microblogger wrote.
"At first I thought I was hallucinating until the copilot asked me whether I saw it too," he wrote. "It was so big that it occupied the whole side window, and the other flights had already reported it as we prepared to do so."
The object faded and finally disappeared 20 minutes later, according to the microblogger.
The phenomenon was likely caused by "astronautic or military activities," Zhu Jin, curator of the Beijing Planetarium, told the Global Times on Tuesday, and was probably not a celestial body.
Zhu did not agree with Web users who defined the object as a "UFO."
"UFO stands for 'unidentified flying object,' while the scene this time is more accurately described as an unidentified aerial phenomenon," he said.
Jin Yuyue contributed to this story