U.S. Secretary of State John Kerry has delivered a stark warning to North Korea not to test-fire a mid-range missile.

Mr Kerry told reporters in Seoul that Pyongyang would only increase their isolation if they launched the missile that US officials believe has a range of some 2,500 miles.

"If Kim Jong Un decides to launch a missile, whether it's across the Sea of Japan or some other direction, he will be choosing wilfully to ignore the entire international community," Mr Kerry told reporters.

"And it will be a provocation and unwanted act that will raise people’s temperatures."

If the trajectory of the test missile suggests that it could be a threat to either the US or allies, the military would move to shoot it down from one of nine warships armed with sophisticated ballistic missile defence systems in the Pacific, including two that were moved closer to the Korean peninsula, US officials said, speaking on condition of anonymity.

Mr Kerry said the test would be a "huge mistake" for Kim.

"It will further isolate his country and further isolate his people who are desperate for food and not missile launches," he warned. "They are desperate for opportunity and not for a leader to flex his muscles."