China has said it will take unspecified measures against a US missile system being deployed in South Korea, and warned Washington and Seoul will bear the consequences.

Beijing "firmly opposes" the deployment of the missile defence system, after US missile launchers and other equipment needed for it arrived in South Korea, a foreign ministry spokesman said at a regular briefing on Tuesday.

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Geng Shuang said China will "definitely be taking necessary measures to safeguard our own security interest".

Mr Geng added "all consequences" resulting from that will be borne by the US and South Korea.

The equipment arrived on Tuesday, a day after North Korea test-launched four ballistic missiles into the ocean near Japan.