North Korea said it is entering a "state of war" with South Korea, a move that seriously escalates a bitter war of words with Seoul and Washington.
North Korea's latest warning comes after the US flew two stealth bombers on a training mission over South Korea during joint military exercises.
A White House official said the US is "taking these threats seriously", while Russia urged restraint.
"We've seen reports of a new and unconstructive statement from North Korea. We take these threats seriously and remain in close contact with our South Korean allies", Caitlin Hayden, a spokeswoman for the White House National Security Council, said.
"But, we would also note that North Korea has a long history of bellicose rhetoric and threats and today's announcement follows that familiar pattern", she added.
North Korea has threatened to close an industrial centre which is one of the two joint enterprises still operational in the region. A newsreader on the state broadcaster announced that the Kaesong industrial complex would be closed "without mercy" if America and its allies did not stop "seeking to tarnish the country's reputation".
The UK Foreign Office said in a statement that North Korea should not continue to isolate itself.
– Foreign Office statement
We have made clear to North Korea that its long-term interests will only be served by constructive engagement with the international community. These threatening statements will only seek to isolate it further.
The armistice agreement has enabled the Korean peninsula to benefit from 60 years' peace. Maintaining it is in the best interests of all.
The government in Seoul said it was not unduly concerned by North Korea's increasingly war-filled rhetoric. The country has been threatening to bomb targets in the South and American military bases since the beginning of March.
The Chinese and Russian governments have been quick to condemn any moves by their ally North Korea which would re-ignite the conflict that marred the country in the 1950s.
"We hope that all parties will exercise maximum responsibility and restraint", Grigory Logvinov, a senior Russian Foreign Ministry official, told the Interfax news agency. "And no-one will cross the point of no return", he continued.