Pyongyang this week said it was pulling out of the armistice with South Korea as tensions mount on the Korean Peninsula.
Martin Nesirky, a spokesman for Secretary-General Ban Ki-moon, said the terms of the 1953 agreement don't allow for one side to back out unilaterally.
"The armistice agreement is still valid and in force," he said in a statement.
U.S. State Department spokesman Victoria Nuland described North Korea's statement is "bellicose rhetoric." She added that it wasn't clear what impact the decision would have now because of its focus on a war that halted 60 years ago.
North Korea's nuclear test last month was said by Pyongyang to be part of an anti-U.S. agenda.
South Korean Foreign Ministry spokesman Cho Tai-young echoed Ban's sentiments, saying the agreement was binding to both countries.
Seoul will "strengthen coordination and cooperation with the U.S. and China, and sternly deal with any attempt from North Korea to scrap the armistice agreement," he was quoted by South Korean news agency Yonhap as saying.