The US Pacific Command has started deploying the Terminal High-Altitude Area Defense (THAAD) system in South Korea.
The move follows the latest launch of four ballistic missiles by North Korea, according to media sources.
US Pacific Command commander admiral Harry Harris said: "Continued provocative actions by North Korea, to include yesterday’s launch of multiple missiles, only confirm the prudence of our alliance decision last year to deploy THAAD to South Korea.
"We will resolutely honour our alliance commitments to South Korea and stand ready to defend ourselves, the American homeland, and our allies."
In July 2016, the US and South Korea agreed to deploy the THAAD system to counter North Korean missile threats.
North Korea's continued development of ballistic missile technology is in violation of six UN Security Council Resolutions and poses a threat to the security of South Korea and the entire Asia-Pacific region.
Operated by the US Forces Korea (USFK), the THAAD will be able to shoot down short and medium-range ballistic missiles inside or outside the atmosphere during their final flight phase.
The THAAD deployment is expected to contribute to a layered missile defence that will enhance the alliance's existing missile defence capabilities against North Korean missile threats.
It does not pose any threat to other countries in the region, US Pacific Command stated.