North Korean state media has warned the United States of a "super-mighty pre-emptive strike".
The claim is in response to Secretary of State Rex Tillerson's comments that the United States was looking at ways to bring pressure to bear on North Korea over its nuclear program.
President Donald Trump has taken a hard line with North Korean leader Kim Jong-un, who has rebuffed admonitions from sole major ally China and proceeded with nuclear and missile programmes in defiance of UN Security Council sanctions.
The Rodong Sinmun, the official newspaper of the North's ruling Workers' Party, did not mince its words.
"In the case of our super-mighty pre-emptive strike being launched, it will completely and immediately wipe out not only US imperialists' invasion forces in South Korea and its surrounding areas but the US mainland and reduce them to ashes," it said.
Reclusive North Korea regularly threatens to destroy Japan, South Korea and the United States and has shown no let-up in its belligerence after a failed missile test on Sunday, a day after putting on a huge display of missiles at a parade in Pyongyang.
Mr Tillerson told reporters in Washington on Wednesday that the United States was "reviewing all the status of North Korea, both in terms of state sponsorship of terrorism as well as the other ways in which we can bring pressure on the regime in Pyongyang".
US Vice President Mike Pence, on a tour of Asian allies, has said repeatedly an "era of strategic patience" with North Korea is over.
US House of Representatives Speaker Paul Ryan said during a visit to London the military option must be part of the pressure brought to bear.
"Allowing this dictator to have that kind of power is not something that civilised nations can allow to happen," he said in reference to Mr Kim.
Mr Ryan said he was encouraged by the results of efforts to work with China to reduce tension, but that it was unacceptable North Korea might be able to strike allies with nuclear weapons.
North and South Korea are technically still at war because their 1950-53 conflict ended in a truce, not a peace treaty.
South Korea's acting president, Hwang Kyo-ahn, at a meeting with top officials on Thursday, repeatedly called for the military and security ministries to maintain vigilance.
The defence ministry said US and South Korean air forces were conducting an annual training exercise, codenamed Max Thunder, until April 28. North Korea routinely labels such exercises preparations for invasion.
"We are conducting a practical and more intensive exercise than ever," South Korean pilot Colonel Lee Bum-chul told reporters. "Through this exercise, I am sure we can deter war and remove our enemy's intention to provoke us."