Allies will help combat North Korea: Trump

Australia could be drawn into hostilities with North Korea, with US President Donald Trump announcing via Twitter if China is unable to deal with the rogue nation "the US, with its allies, will".

Mr Trump has publicly ramped up pressure on China to use its influence to curtail North Korean leader Kim Jong Un's nuclear and missile program.

North Korea has responded by threatening nuclear strikes on the US.

Mr Trump's latest tweets also come as the USS Carl Vinson Nimitz-class aircraft carrier-led strike group of warships heads to the Korean Peninsula and two days before US Vice President Mike Pence sets off on a tour of Australia, South Korea, Japan and Indonesia where North Korea will be on the agenda.

"I have great confidence that China will properly deal with North Korea," Mr Trump tweeted on Thursday.

"If they are unable to do so, the US, with its allies, will!"

Mr Trump's inclusion of "allies" differs from a tweet he sent out on Tuesday.

"North Korea is looking for trouble," Mr Trump wrote.

"If China decides to help, that would be great.

"If not, we will solve the problem without them!"

Last week's meetings with Chinese President Xi Jinping at Mr Trump's Mar-a-Lago estate in Florida has soothed previously strained Chinese and American relations.

China on Wednesday surprisingly did not join Russia in vetoing a US-backed United Nations Security Council draft resolution denouncing the gas attack on the citizens of the Syrian town of Khan Sheikhoun.

Since the Syrian civil war began, China has vetoed six resolutions on Syria.

On Thursday Mr Trump told reporters China "has already started" asserting its influence on North Korea by turning back North Korean coal boats.

North Korean coal exports to China are vital for the already struggling regime's economy.

"The vast amount of coal that comes out of North Korea going to China, they've turned back the boats," Mr Trump told the joint press conference with NATO Secretary-General Jens Stoltenberg.

"That's a big step, and they have many other steps that I know about.

"So we'll see what happens.

"It may be effective, it may not be effective.

"If it's not effective, we will be effective, I can promise you that."