The officials met on Saturday and the talks were expected to continue for another day.

Talks in Sweden between U.S. and North Korean officials to iron out agenda and other details of a second summit between their two leaders ended earlier than expected on Monday.

A diplomatic source said, "The two sides were unable to completely narrow their differences over North Korea's denuclearization steps and the U.S.' response. The meeting offered a chance of a full and frank exchange of opinions."

The respective teams were led by Stephen Biegun, the U.S. special representative for North Korea, and North Korean Vice Foreign Minister Choe Son-hui, with Lee Do-hoon from the South Korean Foreign Ministry mediating.

Biegun called on North Korea to take specific denuclearization steps like halting the production of nuclear fuel and weapons, while Choe demanded that the U.S. agree to declare a formal end to the Korean War and drastically ease sanctions.

A diplomatic source said, "Both the U.S. and North Korea proposed terms that the other side found difficult to accept, and it will take at least another round of talks to narrow differences."

The two sides are expected to engage in a flurry of talks in the five weeks or so until the summit.

Choe left the retreat outside Stockholm where the talks were held at around 1 p.m. on Monday and headed to the North Korean Embassy with her suitcase. She did not answer questions from reporters but smiled and thanked Swedish government officials who arranged the meeting.