Beijing on Thursday accused Tokyo of interfering in the South China Sea after Manila said it would lease five Japanese military plans to patrol areas it claims in the heavily disputed waters.

Beijing claims almost all of the strategically vital South China Sea, and is embroiled in a separate row with Tokyo over disputed islands in the East China Sea that has seen relations between the Asian powers sour badly in recent years.

Philippine President Benigno Aquino said Wednesday that Manila would lease five TC-90 training aircraft from Japan to "help our navy patrol our territory", pointing out the disputed area in particular.

China's foreign ministry spokesman Hong Lei said Beijing was "firmly opposed" to challenges to its sovereignty and security and would "remain on high alert".

"Japan is not a party directly concerned in the South China Sea dispute," he added.

"We urge the Japanese side to mind its words and actions and refrain from undermining the peace and stability of the region," he said.

Tensions in the South China Sea -- through which one-third of the world's oil passes -- have mounted in recent months since China transformed contested Spratly reefs into artificial islands capable of supporting military facilities.

China claims almost all of sea, including those Spratly islands currently controlled by the Philippines.

Several other littoral state have competing claims in the region, as does Taiwan.

Aquino -- whose government has infuriated Beijing by taking the South China Sea issue to an international tribunal in The Hague -- is looking to upgrade one of Asia's most badly-equipped armed forces.

Hong's comments came after Chinese Foreign Minister Wang Yi said there were "little grounds for optimism" in Sino-Japanese relations and accused the world's third-largest economy -- a major Chinese trading partner -- of "two-faced" behaviour.

"On the one hand the Japanese government and leaders are constantly claiming they want to improve Sino-Japanese relations, and on the other are constantly making trouble for China everywhere," Wang said Tuesday on the sidelines of the annual meeting of the National People's Congress (NPC), the country's Communist-controlled legislature.

"This is actually a kind of approach typical of a 'two-faced person'."