Top U.S. envoy to North Korea Stephen Biegun has entered Sweden apparently to join North Korean officials for the resumption of talks on Pyongyang's denuclearization after months without dialogue.

North Korea said earlier this week that "preliminary contact" would occur on Friday, followed by "working-level" negotiations on Saturday.

The denuclearization talks have been stalled since the breakdown of a summit in February between U.S. President Donald Trump and North Korean leader Kim Jong Un in Hanoi. The focus will be on whether the two sides can bridge their gaps over denuclearization steps and the possible rewards North Korea would receive for its actions.

North Korea's chief nuclear envoy Kim Myong Gil told reporters before heading to Sweden that he was optimistic about the results, touching on what he called a new signal coming from the United States. He arrived in Stockholm on Thursday.

At the first U.S.-North Korea summit in Singapore in June last year, Kim Jong Un promised to work toward "complete" denuclearization of the Korean Peninsula, while Trump committed to providing security guarantees to Pyongyang.

Their second summit in February in the Vietnamese capital, however, broke down when Kim insisted that a significant portion of the U.N. sanctions against Pyongyang be lifted in return for a partial dismantlement of the country's nuclear program.

The two leaders agreed at their June 30 meeting in the inter-Korean truce village of Panmunjeom that the United States and North Korea would restart working-level negotiations.

The latest development comes even as North Korea engaged in what can be seen as the most provocative act since it started talks with the United States last year -- the test-firing of what it calls a new type of submarine-launched ballistic missile.

Japan, which saw the missile fall into its exclusive economic zone surrounding its territorial waters, has condemned the missile test as a violation of U.N. Security Council resolutions.

North Korea has also repeatedly test-fired short-range ballistic missiles, but Trump has downplayed the significance of such launches.