The PLA Navy launched eight days of naval drills in the Yellow Sea and Bohai Strait yesterday - a day after US Vice-President Joseph Biden departed China for South Korea. The drills were confirmed by a navigation warning issued by the Liaoning Maritime Safety Administration, which oversees the area. The notice banned all civilian vessels from northern portions of the Yellow Sea and Bohai Strait from 4pm yesterday to the same time next Friday. The administration said the area would be used for a military mission, suggesting that the People's Liberation Army would be conducting naval drills similar to those conducted in the area last month. The previous drills ended a day before the Ministry of National Defence announced the creation of an air defence identification zone (ADIZ) over mos of the adjacent East China Sea, drawing protests from Seoul, Taipei, Tokyo and Washington. The zone includes the disputed Diaoyu, or Senkaku, islands, which are claimed by Japan, as well as Taiwan. During his visit to Beijing this week, which was bookended by trips to Tokyo and Seoul, Biden criticised the new defence zone, saying it has caused "significant apprehension in the region". Beijing-based naval expert Li Jie said the drill was scheduled, but might also be intended as a message to countries, such as the United States and Japan, that have conducted military flights through the zone in defiance of Beijing. "It's a fact that the PLA has intensified annual scheduled drills this year," Li said. "As for this drill, I think the PLA wants to send a warning message about the increased flights by the US and Japan that intentionally enter our ADIZ, challenging China's bottom line." Three days after announcing the creation of the air defence zone, the navy sent its first aircraft carrier from its home port of Qingdao to the disputed waters of the South China Sea, parts of which are claimed by Vietnam and the Philippines. Macau-based military analyst Antony Wong Dong said the PLA had become more active because it wanted the country's neighbours to get accustomed to frequent drills. "When the neighbouring countries - especially those small nations that have territorial disputes with China - get used to the PLA's drills around them, they might become numb and uninterested," Wong said. "Then it will provide opportunities for Chinese fleets to make surprise attacks to seize the islets. It has been a common military strategy in China's history." Efforts to strengthen the PLA received a boost after the Communist Party's Central Committee concluded its third plenum last month. The plenum resolution stressed the importance of strengthening the army's fighting capabilities.