The Terminal High Altitude Area Defense (Thaad) system has reached an initial operating capability to defend against North Korean missiles, US officials said on Monday, forging ahead with its deployment despite China's objections.
"We will resolutely take necessary measures to defend our interests," Chinese Foreign Ministry spokesman Geng Shuang told a daily news briefing, without elaborating, Reuters reported.
Beijing argues Thaad's radar could be used to spy into its territory, despite assurances from Washington that Thaad is purely defensive.
US President Donald Trump has urged China to do more to rein in its neighbor's nuclear and missile programmes and recently has been full of praise of Chinese President Xi Jinping's efforts.
The North is technically still at war with the South after their 1950-53 conflict ended in a truce, not a treaty, and regularly threatens to destroy the US, Japan and South Korea.