"This is all about North Korean missiles and the threat that North Korean missiles pose to the Republic of Korea, and it's for the defense of the Republic of Korea and nothing else," Brooks said, as quoted by Yonhap news agency.
Brooks reportedly noted that THAAD gave Seoul and its allies crucial "area defense."
On Monday, Russian and Chinese deputy foreign ministers, Igor Morgulov and Kong Xuanyou, expressed concern over disproportionate US military activities on the Korean peninsula and deployment of the THAAD missile defense systems in South Korea.
China has previously expressed strong opposition to the deployment of THAAD, stating that it undermined Beijing's security interests.
South Korea, in turn, is worried about Pyongyang's continuing nuclear and missile tests. The most recent launch of North Korean ballistic missile took place on Sunday. The ground-to-ground missile reportedly flew some 500 kilometers (310 miles) before falling into the Sea of Japan short of reaching Japan’s exclusive economic zone.