North Korea and the United States remain technically at war because the 1950-53 Korean War, in which Washington sided with the South, ended only with a truce. The letters read "Oppose, Deploy the Terminal High-Altitude Area Defense, or THAAD".
North Korea said Monday that it will close its communication channel with the United States and suggested it will treat detained Americans more harshly to protest new sanctions against leader Kim Jong Un. However, it was unclear if this was over THAAD or in response to Washington naming North Korean leader Kim Jong Un as an individual to be targeted by new sanctions.
"The DPRK will take a physical counter-action to thoroughly control THAAD... from the moment its location and place have been confirmed in South Korea", the artillery bureau of the North's military said in a statement, according to the official Korean Central News Agency (KCNA).
"The U.S. recently impaired the dignity of the supreme leadership of the DPRK by releasing what they call "report on human rights" and "list of targets of special sanctions" related to the DPRK", the outlet stated, calling the sanctions "the most ugly criminal act and a declaration of war against the DPRK".
The so-called NY channel, via North Korea's mission to the United Nations, has been an intermittent point of contact between North Korea and the U.S., which do not have diplomatic relations.
North Korea warned of the "unwavering will of our army to deal a ruthless retaliatory strike" if the USA and South Korea's joint Thaad system is used. The two countries do not have diplomatic ties and their animosities have deepened because of the North's nuclear and missile programs.
In 2011, North Korea used the NY channel to appeal for food aid and in 2012 to warn Washington about a planned satellite launch. He said he would not comment on the details of diplomatic exchanges.
"And North Korea just became a plausible pretext to justify the THAAD deployment", he said.
Along with the sanctions against Kim Jong-un, the USA also blacklisted numerous other high level officials including their security and intelligence ministries. Warmbier's sentence is 15 years of hard-labor.
The North frequently threatens to attack the South and USA interests in Asia and the Pacific.
There are now two Americans being detained in North Korea, and it is unknown how their treatment under "wartime laws" might change. Outside analysts say Pyongyang often attempts to use foreign detainees to wrest concessions from other countries.
"The Yeongnam [southeast] area came up in talks between South Korea and the U.S. as a potential location for THAAD deployment", said one government official.
Being the North's former war ally and largest benefactor, China has done little to rein in the state, only calling for "calmness" whenever tension rises on the Korean Peninsula.
According to the statement, North Korea's military, which has "sufficient latest offensive strike means", will take "more merciless and powerful successive corresponding measures against the USA keen to ignite a war by deploying THAAD".
Still, Kim dismissed Pyongyang's threat as a routine statement - the kind Seoul and Washington have heard repeatedly. It has regularly warned it would do that since 1994.
China and Russian Federation, as well as North Korea have expressed strong opposition to the deployment of THAAD and urged Washington and Seoul to put a stop to it.
There is also concern that THAAD could drive China and Russian Federation to take a softer stand on maintaining sanctions against North Korea for its nuclear program and continued missile tests.
The two Koreas are divided along the world's most heavily fortified border.
Furthermore, the North Korea government said it would handle all matters with the United States - including the treatment of American prisoners - under the precepts of wartime law.