North Korea's military has threatened to take "physical action" after the United States and South Korea announced that they would deploy a sophisticated missile defence system on the Korean peninsula.
Seoul and Washington said on Friday that the Terminal High Altitude Area Defence (THAAD) anti-missile system would be used to counter Pyongyang's growing nuclear and ballistic missile capabilities.
"There will be physical response measures from us as soon as the location and time that the invasionary tool for US world supremacy, THAAD, will be brought into South Korea are confirmed," the North's military said in a statement on Monday.
"We once again warn the enemies that it is the steadfast will of the KPA [Korean People's Army] to make merciless retaliatory strikes to reduce South Korea to a sea in flames, debris once an order is issued."
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Pyongyang test fired what appeared to be a submarine-launched ballistic missile a day after the announcement by Seoul and Washington, sparking swift international condemnation.
The North frequently threatens to attack the South, as well as US interests in Asia and the Pacific.
The planned deployment of the powerful anti-missile system has also angered the South's neighbours, including China, which said on Friday that the move would "seriously damage" regional security in northeast Asia.
South Korean President Park Geun-Hye, however, defended the move as a "purely defensive" action aimed at protecting the South.
"The international community will be aware that we have no intention to target or threaten another country ... we are taking a purely defensive measure to protect our country and our people," Park said in a meeting with advisors.
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A South Korean defence ministry official told the Reuters news agency that the selection of a site for THAAD could come "within weeks", and the allies were working to have it operational by the end of 2017.
It will be used "to protect alliance military forces," Seoul and Washington said on Friday.
The US maintains 28,500 troops in South Korea, a legacy of the 1950-53 Korean war.