China on Monday firmly opposed the affirmation made by the US and Japanese leaders that the US-Japan security treaty covered China's disputed islands in the East China Sea.

"China expressed grave concern at and firm opposition to the statements made by Japan and the United States," Foreign Ministry spokesperson Geng Shuang was quoted as saying by Xinhua news agency. 

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"We firmly oppose that Japan asked for the endorsement of the United States on its illegal territorial claim in the name of the US-Japan security treaty." he added. 

China and Japan lay claims to the disputed islands in the East China Sea. While China calls them as Diaoyu, the Japanese have named them Senkaku. 

A joint US-Japan statement issued over the weekend after Japanese Prime Minister Shinzo Abe's meeting with US President Donald Trump in Washington affirmed that a clause of the US-Japan security treaty covered the Diaoyu islands.

"Diaoyu island and its affiliated islets are China's inherent territory. No matter what anyone says or does, the fact that the Diaoyu islands belong to China cannot be changed," Geng said.

"China will never waver in its determination and will to safeguard national sovereignty and territorial integrity," he added.

He said Japan and the United States should speak and stop making false statements so as to avoid complicating relevant issues and casting a shadow on regional peace and stability.

The joint US-Japan statement also mentioned China's reef construction in the South China Sea. 

Geng said that China's construction on its own islands was "totally within China's sovereignty and has nothing to do with militarisation".

Geng said the leading cause of militarisation in the South China Sea was that some countries had sent aircraft and vessels to show military force or drive a wedge between countries in the region.

He said China had indisputable sovereignty over islands in the South China Sea and its adjacent waters.

He added that China was committed to solving differences with countries directly concerned in a peaceful way, and safeguarding stability in the region with ASEAN countries.

"We urge the United States and Japan to view the South China Sea issue objectively and rationally and do more to contribute to the peace and stability of the South China Sea, not the other way around," Geng said.