Three of China’s five main military regions have mounted air combat readiness exercises as tensions rise with the United States in the South China Sea , state media reported on Tuesday. One aircraft was also sent to warn off other planes in the area, according to an online news service. Haike News, a news app operated by the overseas edition of Communist Party mouthpiece People’s Daily , posted a video saying the Southern Theatre Command sent various aircraft, including an Su-30 fighter and an aerial refuelling tanker, to Subi Reef, a South China Sea atoll that China expanded into an artificial island. The exercise lasted more than 10 hours and also involved mid-air refuelling, the video said, without saying when the drill took place. “Our aim is not to push the limit or break a record. All our exercises are aimed at [preparing for] actual combat,” it said. Subi Reef, in the Spratly Islands, is controlled by China but also claimed by Vietnam, the Philippines and self-ruled Taiwan, which Beijing considers a renegade province. In a second video released by Haike News on Tuesday, a Chinese air force pilot warned another aircraft from entering “Chinese territorial airspace”. “Leave immediately! Otherwise you would be intercepted,” the pilot was quoted as saying in the video. In another sign of preparations for military action, the Southern Theatre Command – which oversees China’s armed forces operations in the South China Sea – sent two bombers to an undisclosed area for day and night combat training, state-run China National Radio reported on Tuesday. Just late last month, the defence ministry said a number of Chinese bombers simulated night take-offs and long-range raids on targets in the South China Sea to boost combat readiness. The Eastern and Northern theatre commands have also mobilised aircraft “recently”, according to a separate video released by the People’s Daily on Tuesday. According to the video, fighter jets, bombers, a naval reconnaissance aircraft and an early warning aircraft were dispatched, with at least one J-10 fighter conducting a night exercise. In addition, China’s J-15 carrier-based fighter aircraft completed mid-air refuelling at night for the first time, the People’s Liberation Army said on its social media account last week. “It marks another breakthrough in China’s carrier-based fighter jets’ ability to conduct all-weather and long-range combat operations,” the post said. The flurry of air force exercises comes amid increased US activity the South China Sea last month. The US sent 67 large reconnaissance planes to the South China Sea in July , up from 35 in May and 49 in June, according to the South China Sea Strategic Situation Probing Initiative, a Beijing-based think tank which operates out of Peking University’s Institute of Ocean Research.