A North Korean official says his country is ready for war if the United States attacks as a US aircraft carrier group heads for the region. There is speculation that Pyongyang may conduct a sixth nuclear weapons test on Saturday.

Vice Foreign Minister Han Song-ryol told the Associated Press that North Korea changed its military strategy two years ago when reports emerged of US-South Korea "decapitation-strike" training to stress pre-emptive actions of its own.

"We've got a powerful nuclear deterrent already in our hands, and we certainly will not keep our arms crossed in the face of a US pre-emptive strike," he said. "Whatever comes from the US, we will cope with it. We are fully prepared to handle it."

He vowed North Korea will keep building up its nuclear arsenal in "quality and quantity".

READ MORE: North Korea says it's not afraid of US military strike

Tensions between Pyongyang and Washington go back to the 1950-53 Korean War, which ended in an armistice, not a peace treaty.

But the heat has been rising rapidly since US President Donald Trump took office in January.

This year's joint war games between the US and South Korean militaries are the biggest so far.

The USS Carl Vinson aircraft carrier has been diverted back to the waters off the Korean peninsula after heading for Australia , and US satellite imagery suggests the North could conduct another underground nuclear test at any time.

China said on Friday tension over North Korea had to be stopped from reaching an "irreversible and unmanageable stage", while Japanese media have said the government in Tokyo is also discussing how to cope with a possible flood of North Korean refugees.

Trump has threatened that if China is not willing to do more to squeeze the North over its nuclear and missile programmes, the US might take matters into its own hands.

READ MORE: China - Military force won't halt North Korea threat

North Korea's military chimed in on Friday saying it would "ruthlessly ravage" the United States if it chose to attack.

"Our toughest counteraction against the US and its vassal forces will be taken in such a merciless manner as not to allow the aggressors to survive," North Korea's official KCNA news agency quoted its military as saying in a statement.

North Korea recently tested a ballistic missile and claims it is close to perfecting an intercontinental ballistic missile and nuclear warhead that could attack the US mainland.

Many analysts believe at its current pace of testing, North Korea could reach that potentially game-changing milestone within a few years - under Trump's watch as president.

The North conducted two nuclear weapon tests last year alone. The first was of what it claims to have been a hydrogen bomb and the second was its most powerful ever.

Expectations are high that the North may put its newest missiles on display during Saturday's massive military parade that marks the "Day of the Sun", the 105th anniversary of the birth of state founder Kim Il-sung.

Han also did not rule out the possibility of a new nuclear bomb test in the near future.

"That is something that our headquarters decides," he said. "At a time and at a place where the headquarters deems necessary, it will take place."

READ MORE: North Korea and mounting tensions - The view from Seoul

Annual US-South Korea military exercises have consistently infuriated the North, which views them as rehearsals for an invasion. Washington and Seoul deny that.

Han said Trump's Twitter statements have also added fuel to the flames.

Trump posted a tweet on Tuesday in which he said the North is "looking for trouble" and reiterated his call for more pressure from Beijing, North Korea's economic lifeline, to clamp down on trade and strengthen its enforcement of UN sanctions to persuade Pyongyang to denuclearise

"Trump is always making provocations with his aggressive words," Han said. "It's not the DPRK but the US and Trump that makes trouble."