A three-ship flotilla with China's South China Sea fleet has conducted combat drills and patrols off the Xisha Islands, as part of its annual offshore training in the South China Sea, the West Pacific Ocean and the East Indian Ocean.

The flotilla left a military port in South China's Hainan Province on Monday, equipped with amphibious landing craft the Changbaishan and the destroyers Wuhan and Haikou.

An advanced weapon system is equipped on the Changbaishan, China's largest landing ship by gross tonnage. Both destroyers have been through major drills.

The drill focuses on testing the operation of lead ships, submarines and aviation forces in combat, and exploring the methods of open sea drills, Jiang Weilie, commander of the South China Sea fleet, was quoted as saying in the People's Liberation Army Daily on Thursday.

Since setting off, the flotilla has been in a full-time combat state. Observation posts along the coast, aviation forces and submarines training underwater have all become imaginary enemies for the flotilla.

"Battle with those 'invisible opponents' will improve our combat drills," said Zhang Hanchuan, training group leader of the flotilla.

Previous combat drills had a specific schedule with the time of the battles, Zhang said, "But now we just pull our forces onto the sea without telling each other the positions, and we need to detect and attack on our own."

The flotilla conducted landing training on Wednesday under a more challenging condition, Hu Dehai, a deputy brigade commander, told the Xinhua News Agency.

"The wind power and the wave heights were all beyond the standard of regular training," Hu said.

The regular drill, however, triggered criticism from the Philippines, claiming that it could inflame territorial disputes in the South China Sea.

Raul Hernandez, foreign ministry spokesperson of the Philippines, claimed that the patrol extends beyond China's exclusive economic zone (EEZ) as mandated by international law, local media GMA News reported Wednesday.

"Under international law, including the United Nations Convention on the Law of the Sea, China's EEZ cannot extend beyond 200 nautical miles measured from the Chinese mainland and the Hainan Island," he said.

"The Philippines is playing weak to draw the world's attention by accusing China's routine drill," Li Jie, a navy military expert, told the Global Times on Thursday, adding that the country has expedited its process in buying marine warships and improving their ability in patrolling the South China Sea.

"Besides, the Philippines is also trying its best to resume the US army's deployment in the country," Li added.