China's navy has carried out a rare joint exercise, involving its three fleets, in the South China Sea as regional tensions over territorial disputes mount. The combined drill was carried out in southern waters by warships, submarines and the naval air force from the People's Liberation Army's North Sea, East Sea and South Sea fleets, national broadcaster CCTV reported on Saturday. Meanwhile, three Chinese marine surveillance ships had monitored and obtained evidence before "expelling" boats manned by Japanese right-wing activists from waters near Diaoyu islands, or Senkakus as they are known in Japan, China's State Oceanic Administration said yesterday. Japan's Foreign Ministry protested to China over the five-hour sailing by three Chinese official ships in the waters, the Kyodo news agency said yesterday. The Kyodo report denied the activists' ships had been driven away. Chinese analysts said the South China Sea drill by the three fleets, which finished on Friday, was the first since 2010 when Beijing staged a similar exercise to protest against the involvement of the aircraft carrier USS George Washington in a joint anti-submarine drill with the South Korean navy in the Yellow Sea. Last week's PLA Navy drill was intended to send a signal to the US and the Philippines, at a time when the carrier USS Nimitz was in the South China Sea, and amid a standoff between China and the Philippines over the Second Thomas Shoal, or Ayungin as it called in the Philippines, in the disputed Spratly islands. "The joint military drill also meant to tell the Philippines that Beijing's tough stance towards them will continue," said retired PLA Colonel Yue Gang. Meanwhile, Xinhua reported yesterday that another group of warships with the North Sea Fleet had left their port in Qingdao , Shandong province, for blue-water training in the western North Pacific Ocean.