WASHINGTON -- Negotiations to dismantle North Korea's nuclear weapons program "may take some time," US Secretary of State Mike Pompeo said Wednesday.

Pompeo's assessment comes about a month after US President Donald Trump and North Korean leader Kim Jong-un agreed at their summit in Singapore that Pyongyang would work towards the "complete denuclearization" of the Korean Peninsula in exchange for security guarantees from the US.

"There's a lot of work to do. It may take some time to get where we need to go," the top US diplomat said at a Cabinet meeting led by Trump.

Pompeo visited Pyongyang earlier this month in a bid to flesh out the deal that critics say lacked a concrete commitment from North Korea to dismantle its nuclear and ballistic missile programs.

He later said the talks were "productive," but also acknowledged that a lot of work remained to be done, including on the issue of setting a deadline for denuclearization.

Trump, who after meeting with Kim declared that the North Korean nuclear threat was over, has since dialed down any expectations of a quick solution to the problem.

On Tuesday he said there was "no time limit" on the process.

And on Wednesday he tweeted there is "no rush, the sanctions remain!"

Pompeo echoed those remarks, saying the negotiations "will be taking place against the backdrop of continued enforcement of existing sanctions."

He also said North Korea is expected to repatriate the remains of some American soldiers killed in the 1950-53 Korean War in the coming weeks.

"I think in the next couple weeks we'll have the first remains returned. That's the commitment," he said.

Recovering and returning the remains of American troops was one of the agreements reached by Trump and Kim at their summit.

A US news report said earlier that North Korea has agreed to return up to 55 sets of remains, possibly on July 27, the anniversary of the signing of the armistice that ended the war. (Yonhap)