The deployment of the THAAD (Terminal High Altitude Area Defence) system announced by the US and South Korea would "seriously damage the strategic security interests of countries in the region including China", Beijing's foreign ministry said in a statement posted online.
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"China strongly urges the US and South Korea to stop the 'THAAD' anti-missile system deployment process," the statement said, adding the system would not aid the "denuclearisation of the Korean peninsula".
China is North Korea's largest trading partner and has been its key diplomatic protector for decades. But relations have soured over Pyongyang's nuclear tests and long-range missile launches, with Beijing supporting UN sanctions.
Still, analysts say it is concerned over any build up of US power on its doorstep, and values Pyongyang as a strategic bulwark.
Chinese President Xi Jinping stressed the importance of "friendly relations" to a visiting North Korean official last month, even after the envoy reportedly told Chinese officials Pyongyang would continue with its internationally condemned nuclear programme.
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The US and South Korea began talks on deploying the THAAD system to the Korean peninsula in February, when the North launched a long-range rocket following a fourth nuclear test in January.
They announced Friday that they had agreed to install it.
Separately, Washington earlier this week imposed sanctions on North Korean leader Kim Jong-Un, calling him directly responsible for a long list of serious human rights abuses.
China condemned the move, with foreign ministry spokesman Hong Lei telling a briefing Thursday that it opposed "public pressure, confrontation and one country's imposing unilateral sanctions on another country by citing domestic laws".
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