It warned that the move threatened to cause a misjudgment and even an accident, and was a blatant act of provocation, globaltimes.cn wrote.
The US military aircraft faced the risk of being mistakenly shot down when it entered a previously announced military drill restriction zone, and the PLA exercised great restraint not to do so, Chinese experts said.
The PLA has shot down five U-2 aircraft in the past as they trespassed into the Chinese mainland airspace, Chinese military observers noted.
The U-2 high altitude reconnaissance aircraft trespassed into a no-fly zone that was hosting a live-fire drill by the PLA Northern Theater Command on Tuesday, which seriously interrupted the Chinese troops' normal exercises, said Defense Ministry spokesperson Wu Qian in a statement.
Wu did not elaborate where exactly the plane trespassed.
According to navigation restriction notices released by Chinese maritime safety authorities in the past days, multiple live-fire drills were being held on Tuesday in the Bohai Sea and Yellow Sea, which are under the jurisdiction of the PLA Northern Theater Command.
The move seriously violated the code of conduct for the safety of air and maritime encounters between China and the US, as well as related international norms, and could have easily caused misunderstanding and misjudgment, and even triggered an air and maritime accident, Wu said.
This was a blatant act of provocation, and the Chinese side is strongly against this, and has lodged solemn representations with the US, Wu said, noting that China demands the US to stop such provocative moves at once, and take concrete actions to maintain peace and stability in the region.
Song Zhongping, a Chinese military expert and TV commentator, told the Global Times on Tuesday that the U-2's trespassing into PLA drill restriction zones risked being mistakenly shot down.
If the U-2 entered Chinese airspace, it fits into norms and logic that the PLA has the right to shoot it down, Song said, noting that even if it did not fly into Chinese airspace, as long as it trespassed into the PLA's previously announced no-fly zone, it would also face the risk of being shot down.
The US should understand that the PLA must have exercised great restraint not to shoot it down, and that it should not provoke again, Song said.
According to publicly available information, the PLA has shot down as many as five U-2s between 1962 and 1967 as they trespassed into Chinese mainland airspace.
Developed by US arms company Lockheed Martin in the 1950s, the U-2 strategic reconnaissance aircraft can conduct spying missions at an altitude of 27,430 meters, with a top speed of 821 kilometers an hour.
The military in the island of Taiwan had been using U-2s to conduct reconnaissance missions on the Chinese mainland since January 1962, and the PLA Air Force shot down the first U-2 on September 9, 1962 in the suburbs of Nanchang, East China's Jiangxi Province.