Intervention

Many experts doubt Kim's sincerity – a nuclear deterrent to US intervention has long been a strategic goal of his isolated, autocratic regime – and few expect this to be a quick process, even if Washington wants results within a year.

Kim Jong Un made a broad commitment to "work toward denuclearisation of the Korean Peninsula" in Singapore but offered no details of how or when North Korea might dismantle a weapons programme that Trump has vowed will not be allowed to threaten the United States.

Nauert said Pompeo had been "very firm" on three basic goals: the complete denuclearisation of North Korea, security assurances, and the repatriation of US remains from the 1950-53 Korean War.

She said there had been no softening in the US positions, although she would not explain why the department no longer defines its aim as "complete, verifiable, irreversible denuclearisation" (CVID).

"Our policy hasn't changed," she said several times when asked about CVID. "Our expectation is exactly what the president and Kim Jong Un jointly agreed to in Singapore, and that is the denuclearisation of North Korea."

Trump committed in Singapore to providing "security guarantees" to North Korea and Washington later called off one of its major joint military exercises with South Korea, which Pyongyang regularly denounces as rehearsals for invasion.

Nauert said US and North Korean officials had set up working groups to deal with "nitty gritty stuff", including verification of efforts to achieve denuclearisation, which would be headed on the US side by Sung Kim, a Korean-American who is also ambassador to the Philippines.

North Korea's official KCNA news agency said Pompeo's delegation was taking part in high-level talks for implementing the Singapore summit statement but gave no more details.