The US is preparing to announce the "toughest and most aggressive" economic sanctions against North Korea, boosting pressure on the regime during the Winter Olympics, Mike Pence has said.

North Korean leader Kim Jong Un has said the United States should be aware that his country's nuclear forces are now a reality, not a threat.

The US military has flown bombers and stealth jets over the Korean Peninsula in drills with South Korean and Japanese warplanes, three days after North Korea fired a missile over Japan.