And if the North had continued with the nuclear weapon and missile tests of a year ago, even the ultra-hardline administration of the US would have had little recourse given the risk of casualties in Japan and South Korea.
One of the officials said a "sizable number" of remains were expected to be handed over.
Several lawmakers have expressed alarm with plans to halt military exercises.
"It's not irreversible. They could come back if the dialogue loses speed, or if North Korea doesn't live up to its denuclearisation commitment".
Negotiations between the U.S. and North Korean teams are continuing in an attempt to iron out the details of what was a vaguely worded agreement between the two leaders in Singapore.
"I wouldn't expect that at this point", he added.
North Korean leader Kim Jong Un has committed to the USA that North Korea will destroy the Ch'olsan County, North Pyongan province testing site, which sometimes is referred to as the Sohae Satellite Launching Ground, CBS News reports.
Chinese state media quoted Mr Xi as hailing the warming friendship with Mr Kim but included no mention of any specific discussions on making progress on the denuclearisation of the Korean Peninsula.
Mattis was answering reporters' questions at the Pentagon in the aftermath of the vaguely worded agreement, which provided few details about the rogue regime's commitment to denuclearize in return for United States security guarantees, the UK's Guardian reported. "They didn't know I was escaping the country".
That engagement has not, so far, occurred. "The United States will continue to monitor this site closely as we move forward in our negotiations", said the official.
Supreme Court sides with online sales tax
North Dakota - was "flawed on its own terms" and was especially problematic due to the rise of internet retail. Instead, consumers were responsible for send states the necessary taxes - something that most people never do.
U.S. President Donald Trump said Wednesday that North Korea has sent back the remains of 200 American soldiers who were killed in the 1950-53 Korean War.
Moon, who has met with Kim twice in recent months, said the North Korean leader is willing to give up his nuclear program and focus on economic development if he's provided with a reliable security guarantee.
"Even though there is no timeline, there is a de facto one now".
Trump also prematurely claimed the return of remains of USA servicemen missing from the 1950-53 Korean War. It said that those remains that Pyongyang has already identified as U.S. servicemen, thought be number about 200, would be repatriated at once.
"It's only a good deal if they dismantle all the facilities at Sohae and re-employ the scientists in something civilian", she said.
Recovery and repatriation has been happening on and off for decades.
Only about 57,000 people registered with the South Korean Red Cross to meet their separated relatives remain alive, a lot of them aged over 70.
Sanctions against North Korea will remain until the communist nation completely scraps its nuclear program, a senior South Korean official said Wednesday, as President Moon Jae-in urged North Korea to present a plan with concrete steps toward denuclearization.
Mattis said the details had yet to be agreed.
THE FACTS: That's not what his Pentagon chief, Jim Mattis, says.
To his supporters, he said, "Right now you are so safe, and such a great event took place, and all over Asia. they're celebrating the great achievement that we made, because you were the ones that put me here".