Warships attached to a destroyer flotilla with the PLA Navy sail in formation during a recent real-combat training exercise. (eng.chinamil.com.cn/Photo by Shen Yang)

The US sent a spy plane to China's coastal region for close-in reconnaissance just hours before the launch of a Chinese military exercise in the East China Sea on Wednesday, a move experts said on Thursday was provocative and could have led to misjudgments.

China is holding a military task in a large part of the East China Sea from 6 pm Wednesday to 6 pm Sunday, China Central Television (CCTV) reported on Wednesday, citing a navigation restriction notice released by the Maritime Safety Administration of East China's Zhejiang Province.

The notice did not give further details on the exercise, including the training objectives.

Earlier on Wednesday, a US Air Force WC-135R reconnaissance aircraft took off from Kadena Air Base in Okinawa, entered China's East China Sea Air Defense Identification Zone and carried out a close-in reconnaissance operation on China's coastlines from the East China Sea to the Yellow Sea, with the closest range to China's territorial sea baseline being about only 21 nautical miles, the South China Sea Strategic Situation Probing Initiative SCSPI, a Beijing-based think tank, said on its social media account on the day.

The US spy plane likely attempted to conduct reconnaissance on China's upcoming military exercise in the East China Sea at the time, including the equipment to be deployed, a Chinese military expert who requested anonymity told the Global Times on Thursday.

It is provocative and concerning for the US military to come all the way to China's doorstep to peek at China's normal and legitimate military exercise, the expert said.

The US should stop such close-in reconnaissance operations to avoid unnecessary misjudgments and risks of accidents, the expert said.

The WC-135R of the US Air Force is capable of monitoring nuclear weapons and other nuclear-related activities, according to media reports.

It is possible that the US military speculated that the Chinese exercise could have involved nuclear equipment, or the US aircraft could have been deployed to monitor potential nuclear activities by North Korea, Song Zhongping, a Chinese military expert and TV commentator, told the Global Times on Thursday.

If a US close-in reconnaissance attempt poses a security threat, then it would be expelled, Song said.

Song urged the WC-135R to fly near Fukushima and monitor Japan's dumping of nuclear contaminated water, which would be a more meaningful move than coming to spy on Chinese military exercises.