The authorities have learnt what the UFO was after investigations, but it was not the "proper time to publicly disclose" the information because there was a "military connection" to it, the State-run China Daily quoted an official as saying.
The UFO has disrupted air traffic over Hangzhou, capital of China's Zhejiang's province on Wednesday.
The Xiaoshan airport was closed after the UFO was detected at around 9 pm, and some flights were re-routed to airports in the cities of Ningbo and Wuxi, said an airport spokesman.
The airport had resumed since operations and more details will be released after an investigation, he said.
Inbound flights were diverted to the nearby airports in Zhejiang province's Ningbo and Jiangsu province's Wuxi.
Outbound flights were delayed for three to four hours.
A staff member at the airport's information desk said the airport had "no idea" how many flights were "affected by the closure".
Another official daily, Global Times, reported that 'a UFO trailed by a fan-shaped white light appeared last Wednesday in Urumqi, northwest China's Xinjiang Uyghur Autonomous Region'.
It was later identified as debris from an intercontinental missile launched by the United States, Song Huagang, secretary general of the Xinjiang Astronomy Society said.