The United States wants a diplomatic solution to the escalating nuclear crisis with North Korea, Defense Secretary Jim Mattis said Tuesday, toning down the shrill rhetoric between the two countries. 

"We maintain the capability to deter North Korea's most dangerous threats but also to back up our diplomats in a manner that keeps this as long as possible in the diplomatic realm," he said in New Delhi after talks with his Indian counterpart.

"That is our goal, to solve this diplomatically, and I believe that President Trump has been very clear on this issue," the US defence chief said.

President Donald Trump and North Korean leader Kim Jong-Un have become embroiled in a bitter war of words after the North detonated its sixth nuclear bomb and test-fired intercontinental missiles -- saying it needs to defend itself against the threat of a US invasion.

Pyongyang's nuclear weapons ambitions -- including a suggestion last week that it could test an H-bomb over the Pacific -- has increased international fears of conflict.

Alarm over Pyongyang's nuclear and ballistic missile programmes dominated this year's gathering of world leaders at the United Nations General Assembly.

Mattis said the United States was continuing to put pressure on the North's leadership through diplomatic channels. 

"We continue to maintain the diplomatically-led effort in the United Nations. You have seen unanimous United Nations Security Council resolutions passed that have increased the pressure, economic pressure and diplomatic pressure, on the North," he said.