The US military says its controversial Thaad missile defence system is now operational in South Korea.
The system can intercept North Korean missiles although full operational capability is still some months away.
Tensions have been rising around the Korean peninsula, with repeated threats from North Korea and the presence of a group of US warships and a submarine.
North Korea reacted angrily to the latest military exercise, accusing the US of risking a nuclear war.
The rise in tension comes only a day after US President Donald Trump said he would be "honoured" to meet North Korean leader Kim Jong-un, in the right circumstances.
The US announced last week it would activate Thaad, which was not expected to be in use until late 2017, within days.
Thaad, which stands for Terminal High Altitude Area Defence, has been installed at a former golf course in the central county of Seongju, amid angry protests.