North Korea will develop Guam strike plan by mid-August

This comes after the North Korea said it was "carefully examining" plans for a missile strike on the US Pacific territory of Guam.

While Tillerson sought to calm Americans' collective nerves by playing down the nuclear threat level and assuring us that "Americans should sleep well at night", statements by Trump and Mattis provide little in the way of reassurance.

In a statement Wednesday, Mattis warned Pyongyang that its actions could lead to the end of the regime, and destruction of its people.

"We ought not engage in this sort of belligerent talk", he told CNN.

North Korea, which is pursuing missile and nuclear weapons programs in defiance of UN Security Council resolutions, accuses the United States of devising a "preventive war" and has said that any plans to execute this would be met with an "all-out war, wiping out all the strongholds of enemies, including the USA mainland". "Its threat on Guam appears to be aimed at boosting internal solidarity", the official said, according to South Korea's Yonhap news agency.

On Wednesday, Secretary of State Rex Tillerson defended Trump's rhetoric, saying the President was simply adopting a type of language that would resonate more with the North Korean leadership.

These are some of strongest comments Mattis has made on North Korea.

While denouncing the annual Ulichi Freedom-Guardian exercises, North Korea has also dangled the possibility of suspending its nuclear programs if the joint training by more than 25,000 USA and South Korean troops were canceled.

North Korea has repeatedly denounced the THAAD deployment and escalated its threats after the United Nations Security Council voted 15-0 last week to impose tougher sanctions on the North. Trump has said he will not allow Pyongyang to develop a nuclear weapon capable of hitting the US.

The official said the threats would only increase as the U.S., South Korea and several allies prepare for joint military exercises on the peninsula beginning August 21. "The only time our posture goes up is based on facts, not because of what Kim and Trump say to each other", the official added.

Trump's unexpected remarks prompted North Korea to say on Thursday it was finalising plans to fire four intermediate-range missiles over Japan to land 30-40 km (18-25 miles) from Guam, adding detail to a plan first announced on Wednesday. Tens of thousands of U.S. troops remain stationed in South Korea and in nearby Japan.

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