The US and South Korea agreed to install an anti-ballistic missile system on the East Asian peninsula to improve defence capabilities against North Korea, officials said in Seoul Friday.
The decision drew outrage from China, but support from Japan.
It is "part of a defensive action to guarantee the security of the Republic of Korea and our people from North Korea's nuclear weapons, weapons of mass destruction and ballistic missile threats," the Defence Ministry in Seoul was quoted as saying.
Deployment would occur "as soon as possible," it said.
Plans to install the Terminal High Altitude Area Defense (THAAD) system began after Pyongyang's long-range missile test in February, Yonhap News Agency reported.
China expressed "strong dissatisfaction and resolute opposition" with the decision, warning that it may destabilize the Korean Peninsula.
It said the deployment is not conducive to achieving denuclearization and maintaining stability, the ministry said.
Deploying THAAD in South Korea would rather "seriously harm the strategic and security interests and regional strategic balance for those regional countries, including China," it said.
Seoul said the missile system was only aimed at defence against North Korea.
Japan welcomed the deployment, amid growing threats from North Korea's nuclear and missile programmes.
"The advancement of cooperation between the United States and South Korea contributes to regional stability and peace. Our country supports the decision," deputy Chief Cabinet Secretary Koichi Hagiuda said.
An unnamed defence ministry official told Kyodo News agency that the THAAD's radar would enable swift detection of any missile launch by North Korea.
The deployment is "a boost also in terms of advancing defence cooperation between Japan and South Korea," the official told Kyodo.
The THAAD system has been deployed on the US mainland since 2008.