After months of talks
, the two governments said they are in the final stages of recommending a location in South Korea for the system, called Terminal High Altitude Area Defense (THAAD), to be installed.
Yoo Jeh-seung, South Korea's Defense Policy Office head said it was a "defensive measure" to "protect alliance military forces from North Korea's weapons of mass destruction and ballistic missile threats."
"It will be focused solely on North Korean nuclear and missile threats and would not be directed towards any third party nations," Yoo said at a joint press conference attended by U.S. and South Korean officials.
Last month, South Korea said Pyongyang had launched two missiles
over the Sea of Japan, also known as the East Sea, the latest in a series of U.N. violations.
China, Russia voices disapproval
Despite the assurance the weapons system would only be used to counter potential North Korea hostilities, China and Russia objected within hours of the news.
"The deployment of THAAD is of no help to realize denuclearization of the Korean Peninsula, or the maintenance of peace and stability on the Peninsula," a statement from the Chinese Ministry of Foreign Affairs said.
China urged the U.S. and South Korea to halt the deployment process and to "refrain from actions that might damage the interests of China's strategic and security interests."
Russia's Foreign Ministry said the move would "negatively impact global strategic stability," according to official news agency Interfax. The report did not elaborate further.
As North Korea's closest neighbor, China has repeatedly said it views increased U.S. military presence in the region as an affront to its security.
Yvonne Chiu, an expert on military policy and diplomacy at Hong Kong University told CNN in an earlier interview that China was understandably concerned "about having a U.S. made, U.S. run missile system in its backyard" due to the capability of THAAD to reach Chinese territory.
What does THAAD do exactly?
THAAD can intercept incoming ballistic missiles as it enters what is known as their "terminal" phase (when the missile starts to aim downwards, not just in its upward launch trajectory) at incredible speed and altitude.
It's been used for several years by the U.S. military to protect units in places such as Guam and Hawaii from potential attack, and could thwart the short, medium, and intermediate range missiles that North Korea claims it has.
First sanctions against Kim and regime officials
The formal decision to deploy THAAD comes two days after the U.S. placed sanctions personally on North Korean leader Kim Jong Un and 10 other officials for alleged human rights abuses. That is in addition to heavy sanctions against the government for nuclear and missile activity.
The U.S. State Department said it hopes that the naming of specific individuals will dissuade mid- and lower-tier officials from committing abuses out of fear of being blacklisted.
North Korea lambasted the decision, calling it an "open declaration of war against DPRK" and "the worst crime that can never be pardoned," according to the country's state news agency KCNA.