The North has called a US blacklisting of its leader "a declaration of war" and vowed a tough response
North Korea has said it will turn the South into a "pile of ashes" after moves by the US and Seoul to deploy a missile defence system on the peninsula.
Pyongyang said it would make a "physical response" following the deployment of the advanced THAAD missile defence system.
The US and South Korea said the anti-missile system will be used to counter North Korea's growing nuclear and ballistic missile capabilities.
The announcement was the latest move by the allies against the North, which conducted its fourth nuclear test this year.
The North has also launched a long-range rocket, resulting in tough new UN sanctions.
"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.
"It is the unwavering will of our army to deal a ruthless retaliatory strike and turn (the South) into a sea of fire and a pile of ashes the moment we have an order to carry it out," the statement carried by the official KCNA news agency said.
The North frequently threatens to attack the South and US interests in Asia and the Pacific.
South Korea's Defence Ministry spokesman Moon Sang-gyun warned the North not to take "rash and foolish action".
Otherwise, he said, it would face "decisive and strong punishment from our military."
The move to deploy the THAAD system came after the US blacklisted North Korean leader Kim Jong-Un for human rights abuses.
North Korea called the blacklisting "a declaration of war" and vowed a tough response.
Pyongyang said it had notified America that it would sever the only channel of diplomatic communication between them.
It will handle all matters, including any discussions about two Americans it has detained, under wartime laws, it said.
South Korean President Park Geun-hye said the THAAD system was not intended to target any third country.
He said it was purely aimed at countering the threat from the North, in an apparent message to Beijing.
Chinese Foreign Minister Wang Yi said that THAAD exceeded the security needs of the Korean peninsula.
He suggested there was a "conspiracy behind this move."