The US President Donald Trump on Thursday said that perhaps his threat to visit "fire and fury" upon North Korea for its provocations "wasn't tough enough," despite sparking worldwide fears of a potential nuclear conflict.

Trump reaffirmed his threats against the regime of Kim Jong-un in remarks to reporters before a security meeting with Vice President Mike Pence, national security adviser H.R. McMaster and Cabinet chief John Kelly, Efe reported.

On Tuesday, Trump had said that if North Korea continued its provocations it would face "fire and fury like the world has never seen," a comment that prompted Pyongyang to threaten to attack the island of Guam, a US territory where Washington maintains an important naval base.

Trump maintained his hardline stance on Thursday, saying that if North Korea launched an attack, "things will happen to them that they never thought possible."

Nevertheless, the president left the door open to diplomacy, but he also said that for 25 years attempts at dialogue with Pyongyang's communist regime had failed and "it's about time that somebody stuck up for the people of this country."

With Pence at his side, before going into the meeting at his Bedminster, New Jersey, golf club, where he is on an extended vacation, Trump said that he is supported "100 per cent by our military" and by other world leaders.

Trump expressed thanks to China and Russia, traditional allies of Pyongyang, for voting last weekend in favour of new sanctions in the UN Security Council in response to the two most recent North Korean ICBM tests in July.

Trump said he did not want to comment upon the possibility that Washington could stage a preemptive strike on North Korea, which on Thursday said it was finalizing plans to launch two medium-range missiles into the exclusive economic zone off the coast of Guam.

'N.Korean missiles can reach Guam in 14 minutes'

If North Korea launches missiles on Guam, it would take only about 14 minutes for the rockets to reach the US territory, the island's Homeland Security spokeswoman announced.

Jenna Gaminde told a press briefing on Thursday that the residents would be immediately notified by the 15 All-Hazards Alert Warning System sirens, located in low-lying areas throughout the island, the Pacific Daily News reported.

Gaminde's remarks comes after North Korea announced earlier on Thursday that the nation has a detailed plan for a missile strike near Guam, where about 7,000 US troops are stationed, using four intermediate-range ballistic rockets.

The plan calls for sending 12 rockets over Japan to crash in the waters about 30 miles from Guam, which is 2,100 miles east of the Korean peninsula.

"Our office will be notified from the military and will utilize all forms of mass communication to get the message out to the public.

"Local media, village mayors and social media would be used to disseminate information.

"If you hear the sirens, tune into local media - radio, print, television - for further instructions," the spokeswoman added.