North Korea's military, which has "sufficient latest offensive strike means", will take "more merciless and powerful successive corresponding measures against the US keen to ignite a war by deploying THAAD", it said.
It also warned the South of "miserable self-destruction" as a consequence of deployment of the THAAD system.
"We once again warn the enemies that it is the steadfast will of the KPA to make merciless retaliatory strikes to reduce south Korea to a sea in flames, debris once an order is issued," the statement said.
Seoul denounced the "ridiculous threats" by the North, which had staged serious provocations including a nuclear test in January and a long-range rocket launch in February.
"North Korea must recognise who is threatening peace and stability on the Korean Peninsula and first apologise for its provocations," defence ministry spokesman Moon Sang-Gyun told reporters.
'Purely defensive'
Pyongyang also test-fired what appeared to be a submarine-launched ballistic missile a day after the announcement by Seoul and Washington, sparking swift international condemnation.
Saturday's launch followed Pyongyang's back-to-back tests of a powerful new medium-range Musudan missiles on June 22 -- theoretically capable of reaching US bases as far away as Guam.
Tensions are high high since Pyongyang carried out its fourth nuclear test in January, followed by a series of missile launches that analysts said showed the North was making progress toward being able to strike the US mainland.
The planned deployment of the powerful anti-missile system has angered the South's neighbours including China, which said Friday 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.