The controversial Terminal High Altitude Area Defense (THAAD) missile defence system has reached initial operating capability in South Korea, although it is months away from being fully operational, the US military official said.
THAAD, designed to intercept and destroy ballistic missiles during their final phase of flight, has now reached an "initial intercept capability", an official, speaking on the condition of anonymity, told the AFP news agency on Monday.
The missile defence system has been installed at a former golf course in the central county of Seongju, amid angry protests.
However, the official cautioned that THAAD would not be fully operational for a period of months, adding that it will be strengthened later this year as more parts of the system arrive.
THAAD's deployment comes as tension soars on the Korean Peninsula following a series of missile launches by the North, and warnings from US President Donald Trump that military action is an "option on the table".