China said Tuesday it would "resolutely" defend its security interests as the US began deploying the THAAD missile-defence system in South Korea that Beijing fears will undermine its own military capabilities.

"We are firmly opposed to the deployment of THAAD in the Republic of Korea by the US and the ROK," foreign ministry spokesman Geng Shuang told reporters at a regular press briefing in Beijing.

"China will resolutely take necessary measures to defend our own security interests. All consequences entailed from this will be borne by the US and the ROK," he said.