US President Donald Trump says North Korea is "looking for trouble" and has pledged his country will "solve the problem" alone if needed.

Mr Trump took to Twitter on Tuesday night - early morning in the United States east coast - to declare China will get a better trade deal with Washington if they help with North Korea.

"I explained to the president of China that a trade deal with the US will be far better for them if they solve the North Korean problem!" Mr Trump wrote.

"North Korea is looking for trouble. If China decides to help, that would be great. If not, we will solve the problem without them! U.S.A." he continued.

Mr Trump met with Chinese president Xi Jinping at his resort in Florida last week.

On Tuesday, Russia said it was concerned the US would take unilateral action against North Korea.

North Korea also threatened to launch a nuclear attack against the United States if there was any sign of a US pre-emptive strike, as China and South Korea agreed to take new measures against the rogue state if it conducts another nuclear or intercontinental missile test.

The threat came as a US navy strike group was diverted from planned port calls to Australia and moved towards the western Pacific ocean, near the Korean peninsula, in a move one US official described as a show of force.

North Korean official newspaper Rodong Sinmun said the country was prepared to respond to any aggression by the United States.

"Our revolutionary strong army is keenly watching every move by enemy elements with our nuclear sight focused on the U.S. invasionary bases not only in South Korea and the Pacific operation theatre but also in the U.S. mainland," it said.

A statement from North Korea's foreign ministry, distributed by state news agency KCNA, said the navy group's approach showed America's "reckless moves for invading had reached a serious phase".

"We never beg for peace but we will take the toughest counteraction against the provocateurs in order to defend ourselves by powerful force of arms and keep to the road chosen by ourselves," an unidentified ministry spokesman said.

The North convened a Supreme People's Assembly session on Tuesday, one of its twice-yearly sessions in which major appointments are announced and national policy goals are formally approved.

But South Korean officials took pains to quell talk in social media of an impending security crisis or outbreak of war.

"We'd like to ask precaution so as not to get blinded by exaggerated assessment about the security situation on the Korean peninsula," Defence Ministry spokesman Moon Sang-kyun said.

with Reuters