At a news conference on Thursday, the State Department's deputy spokesperson Robert Palladino thanked Vietnam for its generosity in hosting the second summit between US President Donald Trump and North Korean leader Kim Jong Un.
Following what he called a "productive meeting" between a top USA envoy and Pyongyang, Trump tweeted Friday evening that a time and date for a second summit have been "agreed upon".
U.S. Special Representative for North Korea Stephen Biegun is still in Pyongyang in marathon talks with North Korean officials ahead of the second U.S. He said Washington was willing to discuss "many actions" to improve ties and entice Pyongyang to give up its nuclear weapons and that President Trump was ready to end the 1950-53 Korean War, which concluded with an armistice, not a peace treaty.
Months after their Singapore summit, Mr Trump said Mr Kim had written him "beautiful letters".
Meanwhile, the United Nations has warned that North Korea is continuing its nuclear programme and breaking sanctions.
The first ever US-North Korean summit was held in Singapore in June.
South Korea's Yonhap news agency earlier quoted diplomatic sources as saying that Biegun was expected to fly back to South Korea on Friday to share the outcome of his Pyongyang visit, although his stay in North Korea could be extended if additional discussions were needed.
President Trump has been eager for a 2nd Summit despite a lack of significant moves by NKorea to give up its nuclear weapons program. Kim pledged then to work toward the "complete denuclearization" of the Korean Peninsula, without providing a clear timetable or roadmap.
But North Korea said past year that February 8 would be celebrated as the date that the Korean People's Army was founded, as that is the day in 1948 that Kim Il Sung began a transformation that culminated in its establishment.
"North Korea, under the leadership of Kim Jong Un, will become a great Economic Powerhouse", he wrote.
Experts say Kim is determined to win relief from USA -led sanctions to help revive his country's troubled economy, while Trump, faced with domestic problems such as the Russian investigation, needs foreign policy achievements.
Pyongyang has repeatedly urged a lifting of punishing US -led sanctions, a formal end to the war, and security guarantees.
But tough sanctions have limited what they can do, with Washington insisting on keeping up economic pressure until North Korea takes stronger steps toward irreversibly and verifiably relinquishing its nuclear weapons.
But on Friday afternoon, an American military aircraft was seen taking off from the U.S. Air Base in Osan, raising speculation that Biegun could depart by Friday.