The decision to activate the Terminal High Altitude Area Defence (THAAD) system came during talks between the acting South Korean President Hwang Kyo Ahn and Mike Pence, as the US Vice President warned North Korea that the "era of strategic patience is over".
At a joint press conference with Mr Pence today, Mr Hwang said: "We have agreed to further strengthen the readiness posture of [the] ROK-US alliance that matches the threats posed by North Korea through a swift deployment of THAAD."
A failed North Korean missile test on Sunday, when an unidentified projectile exploded moments after being launched in an eastern port city, came in defiance of US President Donald Trump's demands for the country to wrap up its nuclear programme.
Operations to install the THAAD began in early March, the day after the North launched four ballistic missiles, when the United States flew in the "first elements" of the system to a military base near the South Korean capital Seoul.
It has a range of 200km, and is capable of destroying incoming warheads by intercepting them mid-flight. The system has previously been installed in Guam and Hawaii.
Speaking at the time, Chinese foreign spokesperson Geng Shuang said China was "firmly opposed" to the deployment, and would "resolutely take measures to defend our own security interests". He reiterated China's in a press conference earlier today.
Beijing believes THAAD is a threat to its claims in the South China Sea, which it views as its territorial possession. The rest of the world views the region as international waters.
As such, South Korea has been hit with a string of punitive measures. Korean missionaries have been kicked out of the country and K-pop stars have been denied entry, while Korean businesses have faced significant losses, protests and hacking attacks.
Earlier this month, China froze package tours from Beijing to Seoul.
The THAAD was already slated for completion within months, and recent sabre-rattling by all parties has given the construction new urgency.
Earlier this week, Donald Trump's national security advisor said the North Korean "problem" was "coming to a head" and said he believed there was "an international consensus now, including the Chinese leadership, that this is a situation that just cannot continue."
And speaking today at the border of the heavily-mined demilitarized zone which bisects the peninsula, Mr Pence sounded a similar note.
Referring to an "iron-clad alliance" between the United States and South Korea, he said: "All options are on the table to achieve the objectives and ensure the stability of the people of this country."
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