Washington is ready to "ratchet up" pressure on an increasingly aggressive North Korea, US Secretary of State John Kerry said Monday, but remains open to negotiations if Pyongyang scraps its nuclear weapon development.

North Korea has taken a series of actions this year that have ramped up regional tensions, starting with its fourth underground nuclear test in January.

That was followed by the launch of a long-range rocket a month later -- which was widely seen as a disguised ballistic missile test.

In response the UN Security Council slapped its toughest sanctions yet on the secretive state.

"I would like to see a few measures we were not able to get into the (Security Council) resolution implemented, depending on what actions the North decides to take," Kerry told reporters after a Group of Seven foreign ministers' meeting in the Japanese city of Hiroshima.

On Saturday North Korea said it had successfully tested an engine designed for an inter-continental ballistic missile, which it claimed would "guarantee" an eventual nuclear strike on the US mainland.

"So it is still possible we will ratchet up even more depending on the actions" of North Korea, Kerry said.

"But we have made it clear... we are prepared to negotiate a peace treaty" on the Korean peninsula.

"It all depends on the North making the decision that they will negotiate on denuclearisation. We are waiting for that opportunity."

The 1950-53 Korean War ended with an armistice and not a full peace treaty. The US has long insisted that Pyongyang must denuclearise as a condition for talks on a peace pact.

The State Department confirmed in February that Pyongyang had reached out to Washington in a tentative bid to discuss a treaty, but said its January nuclear test had derailed the possible talks.

- 'Absurd' -

Saturday's test was the latest in a series of claims by the North of significant breakthroughs in nuclear weapons and ballistic missile programmes.

They included Pyongyang's alleged success in miniaturising a nuclear warhead to fit on a missile.

Earlier Monday the G7 meeting in Hiroshima, which suffered the world's first nuclear attack in the closing days of World War II, issued a statement calling for a "world without nuclear weapons".

It said North Korea's nuclear ambitions were a key hurdle to achieving that lofty goal.

Kerry also took a swipe at North Korean leader Kim Jong-Un, saying his actions "stand out as such an aberration against the direction the world wants to go" -- referring to moves aimed at reducing nuclear weapons.

"It is also why any suggestion by any candidate for high public office that we should be building more weapons and giving them to a country like (South) Korea or Japan are absurd on their face and run counter to everything that every president, Republican or Democrat alike, has tried to achieve since World War II," he said, apparently referring to Donald Trump.

The Republican front-runner for November's presidential election sparked criticism recently by suggesting that he could accept a nuclear-armed Japan and South Korea to counter North Korea.

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KERRY GROUP