US President Donald Trump has escalated his rhetoric on North Korea, saying his "fire and fury" comment might not be "tough enough."

"Maybe that statement wasn't tough enough," Mr Trump told reporters as he prepared to meet with top national security advisers.

"If anything, that statement may not be tough enough."

It comes after an adviser to Mr Trump said the United States would use "any appropriate measures" to protect itself from threats from North Korea.

The reclusive regime has said it is developing a plan to fire rockets close to US territory Guam.

"Donald Trump has been unequivocal: he will use any appropriate measures to protect the United States and her citizens," Sebastian Gorka, deputy assistant to Mr Trump, told BBC radio.

"We do not telegraph our future scenarios and how we are going to react," he said.

"If you show players around a table your poker hand, you will lose that game. It is not a good idea in cards, it is a very bad idea in geopolitics."

Tensions have risen since North Korea carried out two nuclear bomb tests last year and two intercontinental ballistic missile tests in July.

Mr Trump has said he will not allow Pyongyang to develop a nuclear weapon capable of hitting the United States.

He said that North Korea "will be met with fire and fury like the world has never seen" if it threatened the US again.

North Korea called his warning a "load of nonsense".

It said it was finalising plans to fire four intermediate-range missiles over Japan to land 30-40km from Guam, more than 3,000km to the south.

Guam is home to about 163,000 people and a US military base that includes a submarine squadron, Andersen Air Force Base and a Coast Guard group.

"Sound dialogue is not possible with such a guy bereft of reason and only absolute force can work on him," a report by the North's state-run KCNA news agency said of President Trump.

The army will complete its plans in mid-August, ready for North Korean leader Kim Jong-un's order, KCNA reported.

The report cited General Kim Rak Gyom, commander of the Strategic Force of the Korean People's Army.

While North Korea regularly threatens to destroy the United States and its allies, the report was unusual in its detail.

Masao Okonogi, professor emeritus at Japan's Keio University, said the North Korean reports suggested Pyongyang was issuing a warning or advance notice of changes to its missile testing programme rather than threatening an attack.

"I believe this is a message saying they plan to move missile tests from the Sea of Japan to areas around Guam," he told Reuters.

"By making this advance notice, they are also sending a tacit message that what they are going to do is not an actual attack."

North Korea's apparently rapid progress in developing nuclear weapons and missiles capable of reaching the US mainland has fuelled tensions and erupted into a war of words, unnerving regional powers and global investors.

South Korea urged North Korea to stop all action that is driving up tension on the Korean Peninsula.The South Korean government also said it will seek all methods possible to resolve tensions with the cooperation of other countries.