In addition to North Korea committing "to work toward complete denuclearization of the Korean Peninsula", both countries vowed to recover POW/MIA remains - including the "immediate repatriation" of those already identified by the North.
A U.S. official said Washington was "concerned" by North Korean violations of UN-approved sanctions, including illegal shipments of oil by sea.
The lab will then set about trying to match any American remains with the identities of MIA personnel.
Major-General Kim Do Gyun, the South's chief negotiator in charge of North Korea policy at the Defence Ministry, told reporters he would make efforts to craft "substantive" measures to ease tensions and build trust. "We'll see what happens".
The remains were then moved in gray vans to an airfield where US and South Korean soldiers loaded them one by one into transport planes.
The process of finding and repatriating their remains has always been hampered by North Korea's reluctance to allow US military investigators unrestricted access to battle sites, and to wring as much political capital and money out of the process of returning the remains.
The official, who discussed previously undisclosed aspects of the remains issue on condition of anonymity, said it probably will take months if not years to fully determine individual identities from the remains, which have not yet been confirmed by US specialists to be those of American servicemen. The official, who discussed previously undisclosed aspects of the remains issue on condition of anonymity, also said North Korea provided a single military dog tag along with the remains.
Why are United States remains in North Korea?
"We don't know who's in those boxes", Mattis said, adding they could be missing Americans or Australian or French troops.
Of the roughly 7,700 missing US service members from the Korean War, approximately 5,300 are believed to be on North Korean soil.More news: Sony Unveils World's First 48MP Smartphone Camera Sensor
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In Singapore, Donald Trump met alone with dictator Kim Jung Un before signing a document that had no dates, no details, no guidelines for ending the buildup of nuclear weapons in North Korea.
North and South Korea are among the 45 countries participating in this year's Asian Games.
Image copyright Getty Images Image caption There are thought to be around 5,300 remains of United States soldiers in North Korea Does North Korea have any others collected?
However, officials did note to the Washington Post that another launch facility - the Sohae Satellite Launching Station on the West Coast - was being partially dismantled. But many experts say those are neither irrevocable nor serious steps that could show the country is honest about denuclearization.
On Monday, a senior U.S. official told Reuters that USA spy satellites had detected renewed activity at the North Korean factory that produced the country's first intercontinental ballistic missiles capable of reaching the US.
But Ms. Nauert dodged questions about reports that USA spy satellite photos and infrared imaging have shown recent movement of vehicles in and out of a North Korean facility that previously produced the country's first ballistic missiles capable of reaching the US mainland.
Numerous fallen service members died in North Korea and were buried by their comrades where they fell.
"We've been watching the news, and we've been hopeful that my uncle is among the remains", he said. Indeed, the commitments regarding its nuclear weapons program were so vague as to be nearly meaningless, in my view. Other U.S. service members were captured and placed in prisoner-of-war camps, where many succumbed to starvation, exposure and torture. "So between this and Kangson, we are seeing an expanded and improved nuclear and missile force". In the past, the North has provided bones that in some cases were not human or that were additional bones of US servicemen already identified from previously recovered remains.
It's possible that questions about the identities of the remains could persist for years to come.
The last time United States military men or women were on the ground to teach for remains was 2005. Indonesia guarantees the security of all countries' representatives and contingents, including those of North Korea.