BEDMINSTER, NEW JERSEY (REUTERS) - President Donald Trump ratcheted up his rhetoric towards North Korea on Thursday (Aug 10), saying it should be "very, very nervous" if it even thinks about attacking the United States or its allies, after Pyongyang said it was making plans to fire missiles over Japan to land near the US Pacific territory of Guam.

"The people of our country are safe. Our allies are safe. And I will tell you this: North Korea better get their act together or they're going to be in trouble like few nations ever have been in trouble in this world," Trump told reporters in New Jersey, where he was meeting his national security team.

Far from toning down his words after saying on Tuesday that any threats by Pyongyang would be "met with fire and fury like the world has never seen," Trump said those remarks may not have gone far enough.

"Maybe it wasn't tough enough. They've been doing this to our country for a long time, for many years. And it's about time that somebody stuck up for the people of this country and for the people of other countries," Trump added.

Asked if he would consider a pre-emptive strike against North Korea to deny it the ability to launch a nuclear attack against the United States, Trump said: "We'll see what happens.

"If North Korea does anything in terms of even thinking about attack of anybody that we love or we represent or our allies or us, they should be very, very nervous. I'll tell you what. And they should be very nervous. Because things will happen to them like they never thought possible," Trump added.

The United States and South Korea remain technically still at war with North Korea after the 1950-53 Korean conflict ended with a truce, not a peace treaty.

Tension in the region has risen sharply since the reclusive communist country - which staged two nuclear bomb tests last year - launched two intercontinental ballistic missile tests in July in defiance of world powers. Trump has said he will not allow Pyongyang to develop a nuclear weapon capable of hitting the United States.

North Korea's army will complete plans in mid-August to fire four intermediate-range missiles over Japan to land near Guam, when they will be ready for leader Kim Jong Un's order, state-run KCNA news agency said. The plans called for the missiles to land in the sea 30-40km from Guam.

Trump also said that new sanctions on North Korea approved by the UN Security Council on Aug 5 that could slash by a third the country's US$3 billion (S$4 billion) annual export revenue probably "will not be as effective as a lot of people think it can be, unfortunately".

Trump praised China and Russia for backing the sanctions, but pressed Beijing to do more.

"I think China can do a lot more, yes... And I think China will do a lot more," Trump said at Bedminster, New Jersey, where he is on a working vacation.

Trump said the United States loses a lot of money on trade with China. "It's not going to continue like that. But if China helps us, I feel a lot differently toward trade," Trump added.

RISKS GROW

Experts in the US and South Korea said North Korea's Guam plans ratcheted up risks significantly, since Washington was likely to view any missile aimed at its territory as a provocation, even if launched as a test.

If Pyongyang carries out its threatened show of force and launches missiles towards Guam, it would represent an unprecedented milestone in the already fraught relations between the US and North Korea.

Trump said the US would "always consider negotiations" with Pyongyang, but faulted three of his predecessors - Barack Obama, George W. Bush and Bill Clinton - on their approach to North Korea.

The planned path of North Korea's missiles would cross some of the world's busiest sea and air traffic routes.

Guam, a tropical island more than 3,000km to the south-east of North Korea, is home to about 163,000 people and a strategically located US air base, a Navy installation that includes a submarine squadron, a Coast Guard group and roughly 6,000 US military service members.

"The Hwasong-12 rockets to be launched by the KPA (Korean People's Army) will cross the sky above Shimane, Hiroshima and Koichi Prefectures of Japan," the North Korean report said."They will fly 3,356.7km for 1,065 seconds and hit the waters 30 to 40km away from Guam."

The report did not mention any threat of the use of nuclear missiles near Guam.

"Sound dialogue is not possible with such a guy bereft of reason and only absolute force can work on him," KCNA said, calling Trump's "fire and fury" comment "a load of nonsense".

Former US President Jimmy Carter rebuked officials in both nations over their threatening language.

"The harsh rhetoric from Washington and Pyongyang during recent months has exacerbated an already confrontational relationship between our countries, and has probably eliminated any chance of good faith peace talks between the United States and North Korea," Carter said.

"In addition to restraining the warlike rhetoric, our leaders need to encourage talks between North Korea and other countries, especially China and Russia."

US Senator Lindsey Graham, a key Republican voice on foreign policy, said that based on his conversations with Trump he believes the president would be willing to launch a pre-emptive strike to prevent Pyongyang from launching a nuclear attack on the US homeland.

"If negotiations fail, he is willing to abandon 'strategic patience' and use pre-emption," Graham said of Trump during an interview with radio host Hugh Hewitt. "I think he's there mentally. He has told me this."

"So I'm 100 per cent confident that if President Trump had to use military force to deny the North Koreans the capability to strike America with a nuclear-tipped missile, he would do that," Graham added.