North Korean state media warned the United States of a "super-mighty preemptive strike" after U.S. Secretary of State Rex Tillerson said the United States was looking at ways to bring pressure to bear on North Korea over its nuclear programme.

U.S. 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 U.N. Security Council sanctions.

"In the case of our super-mighty preemptive strike being launched, it will completely and immediately wipe out not only U.S. imperialists' invasion forces in South Korea and its surrounding areas but the U.S. mainland and reduce them to ashes," said. North Korea state media

U.S. Vice President Mike Pence, on a tour of Asian allies, has said repeatedly an "era of strategic patience" with North Korea is over.

U.S. 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 Kim.

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.