The Terminal High-Altitude Area Defense system, otherwise known as Thaad, is designed to protect South Korea against threats from North Korea.
South Korean media said operations to install a Thaad battery had begun yesterday, with some parts flown from the US to an air base near Seoul.
The move comes a day after the North launched four ballistic missiles in violation of existing international sanctions.
But the US-South Korean arrangement has angered not just the North, but also China; with the Chinese foreign spokesman GENG SHUANG saying that China resolutely opposed the deployment.
The US has previously deployed the Thaad system in Guam and Hawaii as a measure against potential attacks from North Korea.
