The US has sent spy planes over a live-fire Chinese military drill – including a U-2 which entered a declared no-fly zone – triggering a protest from Beijing and heightening the risk of an armed conflict. The development followed weeks of increased US activity in the region, with numerous military jets and vessels deployed to keep a close watch on Chinese activity. The Beijing-based think tank South China Sea Strategic Situation Probing Initiative said a US Air Force RC-135S reconnaissance aircraft flew across the South China Sea on Wednesday, during the Chinese military drill. But it said it appeared the plane was on a transfer, rather than a reconnaissance mission. Chinese defence ministry spokesman Wu Qian said a U-2 reconnaissance jet flew without permission over the no-fly zone in the PLA’s northern military region, where the live-fire drills were taking place. “It seriously interfered in normal exercise activities. It seriously violated the code of safe behaviour for air and sea between China and the US and international norms. It easily leads to misjudgment, or could even cause accidents in the sea and air,” Wu said. “The move was an obvious provocation. China firmly opposes such provocative actions and has lodged solemn representations with the US side.” A source close to the Chinese military said the U-2 high-altitude reconnaissance aircraft came from a military base in South Korea, and flew over the Bohai Gulf where the Chinese aircraft carrier the Shandong was taking part in the exercise. China’s Maritime Safety Administration said live-fire exercises were being conducted in waters off the country’s northeastern coast, stretching from the Bohai Gulf to the Yellow Sea. China’s navy drills in 4 regions show ability to counter US, observers say In a statement, the US military said the U-2’s flight was “within the accepted international rules and regulations governing aircraft flights” and that Pacific air force personnel would “continue to fly and operate anywhere international law allows, at the time and tempo of our choosing”. The Lockheed U-2 can fly over 70,000 feet, providing day and night as well as all-weather intelligence gathering. It has taken part in post-Cold War conflicts in Afghanistan and Iraq, and supported several multinational Nato operations. Relations between the US and China have plunged to historic lows, with confrontations widening from trade to ideology. On the military front, US naval forces regularly conduct operations near Taiwan and in the South China Sea , in a challenge to China’s territorial claims in the region. Liu Weidong, a US affairs expert from the Chinese Academy of Social Sciences, said the U-2 flight was more of a gesture by the United States. “By flying the U-2 into China's no-fly zone, the US is telling China that the rules of the game are not decided by Beijing, and ‘I can come if I want’,” he said. “But the plane is not the most advanced reconnaissance aircraft, and not an attacking warplane either. And similar spying activities from the US have happened many times already. I think this is more a political gesture, saying the US can do whatever they like without being constrained by Chinese military power.” South China Sea: the dispute that could start a military conflict Hu Bo, director of the South China Sea Strategic Situation Probing Initiative, said the incident could have led to serious consequences. “It would be a huge trouble if the plane was shot in the live-fire naval exercise. Frictions are bound to occur if distance is not well kept,” he said. The think tank, which monitors US military activity in the region, said a US Army Challenger 650 had also been spotted over the South China Sea on Tuesday and had been “very active this month” with almost 10 recorded sorties so far. In December 2013, when China’s Liaoning aircraft carrier task group was training in the South China Sea, the USS Cowpens missile cruiser sailed close enough to cross into the Chinese flotilla, leaving a Chinese landing ship with no choice but to force the US ship to a stop. The vessels were 50 metres (164 feet) apart at their closest point.