Chinese experts warned that the United States military is "walking on thin ice" by allowing one of its U-2 spy planes to trespass into a no-fly zone over Chinese live-fire military drills, calling the move extremely provocative and prone to misjudgement.

On Tuesday, a Lockheed U-2 high-altitude reconnaissance aircraft trespassed into the no-fly zone that is hosting a live fire drill by the People's Liberation Army Northern Theater Command, Senior Colonel Wu Qian, spokesman of the Ministry of National Defense, said in a statement on Tuesday.

Wu said the trespass had "seriously disrupted China's regular training activities" and violated China-US maritime and flight safety codes and international norms.

"The incident could very easily lead to misunderstanding and misjudgment, and may even lead to an accident," he said. "It is also a blatant provocation that China resolutely opposes."

Foreign Ministry spokesman Zhao Lijian said in a news briefing on Wednesday that China resolutely opposes the US trespass and urged the US to immediately stop such provocative acts and take concrete measures to maintain regional peace and stability.

The US Pacific Air Forces said in a statement on Tuesday that a U-2 flight had been conducted in the Indo-Pacific region and it had operated within the accepted international rules and regulations governing aircraft flights.

"Pacific Air Forces personnel will continue to fly and operate anywhere international law allows, at the time and tempo of our choosing," it added.

The South China Sea Strategic Situation Probing Initiative, a Chinese nonprofit research institution, said a US RC-135S intelligence aircraft entered air space above the South China Sea on Wednesday morning via a flight path above the southeastern waters of Hainan province, where the Chinese military is holding a drill nearby.

Naval drills

According to local maritime safety administrations, China is holding major naval drills in all four of its major adjacent waters, namely the Bohai Sea, the Yellow Sea, the East China Sea and the South China Sea. Since late July, the Chinese military has conducted a total of 22 drills in these waters.

The exercise in the Bohai Sea, which is the northwestern and innermost gulf of the Yellow Sea bounded by the coastlines of Hebei, Shandong and Liaoning provinces, began on Aug 21 and ends on Friday. The drill in the South China Sea began on Monday and ends on Saturday.

On Wednesday, the local maritime authority in Zhejiang province said the Chinese military will conduct "real weapons training" in the East China Sea from Thursday to Sunday, and prohibited ships from entering the training area.

Major General Zhang Shaozhong, a military commentator, said according to international norms, any nation has the right to declare a temporary no-fly zone for military testing and training, and the announcement must be made in public and in advance.

"It is extremely dangerous to deliberately trespass into a no-fly zone after receiving clear notification, because the radars on live missiles may pick up the wrong signal and pursue the wrong target, causing accidental damage," he said on microblogging platform Sina Weibo on Tuesday.

A military historian who spoke on condition of anonymity said seeing a U-2 spy plane trespassing into a Chinese no-fly zone is like seeing a "ghost of the Cold War", because the plane is infamous for its aggressive intelligence gathering capability and had been shot down multiple times in the 1960s.

Developed in the early 1950s, the U-2 is a single-pilot, high-altitude spy plane known for conducting photo-reconnaissance deep into enemy territory and picking up radio signatures from radar, data links and telecommunication equipment from an extreme operating height of 21.3 kilometers.

"It is a notoriously high-profile target that no competent military could allow to roam freely around their borders," the historian said, adding that the plane played a major role in the Cuban missile crisis, the Vietnam War and the Gulf War.

"The recent trespass was in a temporary no-fly zone, not our territorial airspace. It would be a perilous overreaction and escalation if we tried to intercept or shoot the plane down, and the US knows we can't risk this," the historian said.

"But the US military is also walking on thin ice here. Flying a Cold War relic directly in China's face, our military will not take this insult lightly."

Zhu Feng, executive director of the Collaborative Innovation Center of South China Sea Studies, said the US is sparing no effort to stir up tension in the South China Sea and around China, and is hoping to "artificially manufacture a China crisis".

"History has repeatedly shown that the only way to heal a divided US is for it to tackle a serious external threat, so people could rally around the American flag," he said. "We must see through this insidious ploy and remain calm and collected."

Groundless claims

In another development, Foreign Ministry spokesman Zhao on Wednesday rebuked an article by US Secretary of Defense Mark Esper, published in The Wall Street Journal on Monday, saying his claims about the role of the PLA were "totally groundless".

Zhao said China upholds a defensive national defense policy, and the Constitution of the Communist Party of China and the Constitution of the People's Republic of China have made it clear that China follows a path of peaceful development and opposes hegemony.

"Can the US make such a declaration?" Zhao asked.

"Which country owns hundreds of military bases around the world, waged war or conducted military actions illegally against countries such as Iraq, Syria and Libya, and sent vessels and aircraft to flex its muscles in distant waters away from its own territory?" he said.

"Which country is sticking to a Cold War mentality, withdrawing from treaties and organizations, wielding its 'clubs and fists' and undermining global rules wantonly?" Zhao said.

Also, China has sent more peacekeeping troops than any other permanent member of the United Nations Security Council, Zhao said, pointing out that since 1990, the Chinese military has participated in more than 20 UN peacekeeping operations and sent more than 40,000 peacekeeping troops.