The US military has reportedly started moving Terminal High-Altitude Area Defense (THAAD) equipment to the designated site in Seongju county, South Korea, amid protests from local residents.
The planned THAAD deployment, agreed by the US and South Korea in July 2016, is aimed to counter North Korean missile threats.
Operated by the US Forces Korea, the THAAD will be able to shoot down short and medium-range ballistic missiles inside or outside the atmosphere during their final flight phase.
South Korean Defense Ministry was quoted by CNN as saying: "Both South Korea and the US have been working to secure the operational capacity of the THAAD system in preparation for North Korea's advanced nuclear missile threat.
"Therefore, this measure was to secure operational capacity by placing some parts of the available THAAD system at the deployment site."
The transfer of equipment required for THAAD installation has started 51 days after its road-mobile launchers were delivered to Osan Air Base in South Korea, Yonhap reported.
The earlier-than-expected move to relocate equipment comes before environmental inspections are completed.
Nearly 200 protesters arrived at the THAAD site as military trucks carrying the equipment were arriving at the site.
South Korea has deployed 8,000 police personnel to ensure safe delivery of the equipment.
China has also opposed the deployment of THAAD system as it believes that the system will be used to monitor its activities.
Meanwhile, North Korea has reportedly conducted a large live-fire exercise to mark the 85th founding anniversary of the Korean People's Army.