Mike Pompeo, the US Secretary of State, vowed to get North Korea to make specific commitments towards "total" denuclearisation during the first day of his trip to Pyongyang on Friday.
Mr Pompeo is aiming to leave his third trip to the North Korean capital with the promise of concrete action from Kim Jong-un, the North Korean leader, following his summit with Donald Trump, the US president, last month.
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"On this trip, I’m seeking to fill in some details on these commitments and continue the momentum toward implementation of what the two leaders promised each other and the world. I expect that the DPRK [North Korea] is ready to do the same," Mr Pompeo said.
One key aim of the trip is thought to be the return of the remains of US troops killed during the 1950-53 Korean War.
Both sides have suggested Pyongyang is willing to turn over dozens if not hundreds of sets of remains.
Doubts over the North’s intentions have grown amid reports it is continuing to expand facilities related to its nuclear and missile programs and questions over how willing the North Korean dictator is to give up his nuclear arsenal.
Mr Trump has faced criticism for suspending large-scale US military drills with South Korea before failing to receive a firm time frame for the North’s denuclearisation process.
On Friday night South Korean officials claimed North Korea appeared to be developing a new submarine capable of launching nuclear-armed ballistic missiles.
Satellite imagery reviewed by the country’s intelligence officials suggested North Korea is constructing the submarine in a facility on the east coast of the country, the Wall Street Journal reported.
Mr Pompeo met with Kim Yong Chol, a senior official a who has been something of a point-man on Washington negotiations, on Friday for his first round of talks.
"The more you come, more trust we can build between one another," Mr Kim told America’s top diplomat.
It was not clear if Mr Pompeo would meet directly with Kim Jong-un, as he had done previously, but the diplomat reportedly brought with him a CD with the Elton John song “Rocket Man” as a gift.
Mr Trump repeatedly derided Kim as “Little Rocket Man” during heightened tensions between the two leaders, something Kim reportedly raised at their June 12 summit.
According to the South Korean newspaper The Chosun Ilbo, Mr Trump asked Mr Pompeo to bring a copy of the CD and a letter during his subsequent trip to the state.