"There is still an enormous amount of work to do. There have been no nuclear tests or missile tests... but work on making the strategic shift continues," Pompeo said on a visit to New Delhi.

His comments to reporters came after South Korea said that President Moon Jae-in would hold his third summit this year with North Korean leader Kim Jong Un on September 18-20 in Pyongyang.

At a landmark summit between Trump and Kim in June in Singapore, the two leaders pledged to denuclearise the Korean peninsula but no details were agreed.

Washington and Pyongyang have sparred since on what that means and how it will be achieved.

Trump, frustrated with a lack of progress on disarmament, last month cancelled Pompeo's trip to Pyongyang after the North reportedly sent a belligerent letter to the US leader.

Stephen Biegun, newly-appointed US envoy for the North, said last month Kim had promised "final, fully verified denuclearisation" at the Singapore summit.

But Pyongyang has slammed the Washington for its "gangster-like" demands for complete, verifiable and irreversible disarmament.