And they expressed concern that the work is being done without verification.
A 38 North report said it was a first step towards fulfilling a pledge made at the summit and a significant confidence-building step amid growing questions about North Korea's willingness to give up a nuclear weapons programme that threatens the United States.
An official from South Korea's presidential office said Tuesday that Seoul has also been detecting dismantlement activities at the Sohae launch site but did not specify what the North was supposedly taking apart.
A 67-meter launch pad stands on the site along with a large crane that is used to maintain and fix the pad and to set up a long-range missile.
"This is consistent with North Korea's public line, which is that its successful test program is now transitioning to mass production of nuclear and missile systems", he said.
Yonhap, citing an unidentified diplomatic source, said on Thursday North Korea had accepted the caskets, which were carried in two trucks, and was expected to transfer the remains on Friday.
The factory visit was the latest of a series of site inspections by Kim, including industrial facilities and special economic zones near North Korea's border with China, according to state media, after Kim shifted his focus from nuclear missiles to the economy in April.
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"The fact of the matter is that a year ago, on President Trump's watch, North Korea made significant advances in its ballistic missile technology, achieving what many experts agree is a nuclear-capable operational ICBM", Menendez added. "So far, the facilities dismantled have been peripheral to these core functions".
Lee Bo-kyung has more. Reuters news agency later reported it had found out the site was Sohae. "The North can also fire ICBMs from transporter erector launchers, but their technology with these vehicles isn't stable".
On a visit to a military rations factory, Kim said it should produce tasty and nutritious food to "substantially contribute to improving the diet of the servicepersons", according to KCNA.
"Unless they dismantle the whole site, it will remain North Korea´s premier location for space launches", Hanham said on Twitter.
"Dismantling a test site does not seriously constrain the existing arsenal or even future designs", said Mount. "Easing the missile threat would require restrictions on the number, types, or capabilities of missiles or the vehicles that transport and fire them".
Trump told a news conference after his June 12th summit with Kim that the North Korean leader had promised that a major missile engine test site would be destroyed very soon. Likewise, no Nuclear Tests. Kim announced the mission of his nuclear testing site as finished weeks before inviting foreign journalists to observe the destruction of the tunnels.
As Pompeo spoke in California, Trump told an audience of USA veterans in Kansas City he hoped the remains of American soldiers killed in the 1950-53 Korean War would be returned by North Korea soon - another agreement the president has touted from the summit that has been slow to materialise.