Barack Obama's former defence secretary has warned war with North Korea is possible, but it would come with "an intensity of violence" we haven't seen in decades.

Speaking to ABC News' Martha Raddatz on This Week, Ash Carter said he believed a pre-emptive strike on Pyongyang could see Kim Jong-un attempt to invade South Korea.

While Carter said he was confident the US would emerge victorious, the resulting war would be violent and intense.

"It's quite possible that they would as a consequence of that launch ... attempt an invasion of South Korea," he told Raddatz.


"As I said I'm confident of the outcome of that war, which would be the defeat of North Korea.

"But, Martha, I need to caution you, this is a war that would have ... an intensity of violence associated with it that we haven't seen since the last Korean War."

Carter, a physicist and former Harvard University professor of Science and International Affairs, also called for caution in any approach taken with North Korea.

"Seoul is right there on the borders of the DMZ, so even though the outcome is certain, it is a very destructive war," he said. "And so one needs to proceed very carefully here."

Carter said the Obama and previous US administrations had appealed for China to put more pressure on Pyongyang to end its nuclear weapons program.

However he said China was reluctant to do so in case it led to war or the collapse of North Korea which would result in a unified Korea and its allies on its border.

Meanwhile US President Donald Trump said the US was prepared to act unilaterally to deal with North Korea's nuclear program if China didn't.

"Well, if China is not going to solve North Korea, we will. That is all I am telling you," he said in an interview with the Financial Times of London.

Trump's strong words come ahead of a meeting later this week with Chinese President Xi Jinping at the US president's Mar-a-Lago resort in Florida.

It will be the first face-to-face meeting between the two leaders.

"China has great influence over North Korea. And China will either decide to help us with North Korea, or they won't," Mr Trump told The Times.

"If they do, that will be very good for China, and if they don't, it won't be good for anyone."

In a separate interview, US ambassador to the United Nations Nikki Haley said the United States is looking to China to take action against North Korea.

"The only country that can stop North Korea is China and they know that," Haley told ABC's This Week in an interview on Sunday.

"We're going to continue to put pressure on China to have action."

Haley emphasised that at the Florida meeting "the most important conversation will be how we're going to be dealing with the nonproliferation of North Korea."

Beijing, increasingly frustrated with Pyongyang's nuclear and missile activities, has announced a suspension of all coal imports from the North until the end of the year.

"At some point, we need to see definitive actions by China condemning North Korea and not just calling them out for it," she said.

Tensions have risen sharply as North Korea has stepped up ballistic missile tests and amid boasts Kim Jong-un that his country was in the final stages of developing an intercontinental ballistic missile.

US-based analysts have warned that North Korea appears to be preparing a new nuclear test.

Pyongyang has staged five nuclear tests so far and two last year.

Since taking office, Trump has left open the possibility of military action against North Korea.

Following that country's early March missile tests, which came provocatively close to Japan, the US leader emphasised his administration's commitment to "deter and defend against North Korea's ballistic missiles using the full range of United States military capabilities."

In February US defence Secretary James Mattis reassured Washington's Asian allies and said any nuclear attack by North Korea would trigger an "effective and overwhelming" response following President Trump's inauguration.


Speaking to last month regarding how Trump will handle the North Korean issue, Brendan Thomas-Noone a research fellow at the United States Studies Centre at the University of Sydney, said conflict was the last thing the region needed.

Thomas-Noone warned pre-emptive strikes or sanctions will not result in North Korea restricting its nuclear program or its reach for ICBM technology in the long-term.

"A conflict on the Korean peninsula would end up engulfing the entire region," he said.

"It's unpredictable how a conflict or crisis could unfold, and with so many overlapping alliances and US defence commitments in the region, a stable Korean peninsula is in the interest of the entire region, including Australia."