The Washington Post reported on Monday that USA intelligence agencies are seeing signs that work is under way on one or two liquid-fueled ICMBs at the Sanumdong research facility, according to officials who spoke on the condition of anonymity.
But he insisted the Trump administration was still making progress in its talks.
White House counselor Kellyanne Conway on Tuesday played down the significance of the Post report.
Melissa Hanham, a North Korea expert at the Middlebury Institute of International Studies, tells the BBC that satellite images show there has been "regular traffic in and out of the building" at the Sanumdong facility-and the pattern didn't change throughout peace talks in Singapore and the DMZ. "Things don't change overnight", Conway later added.
An Indonesian delegation led by Coordinating Human Development and Cultural Affairs Minister Puan Maharani delivered a personal invitation to Kim during their visit to North Korea this week.
North Korea launched a wave of ballistic missiles from February 2016.
US Secretary of State Mike Pompeo last week said that North Korea was continuing to produce fuel for nuclear bombs.
North Korea got respect, trade, and weapons, and now they appear to be going on as if Singapore put no restrictions on them.
Pixel 3 XL leaks show Google's new notch and possible wireless charging
Of note is that apparently the back features a single piece of glass, but the top part is glossy, while the rest is matte. With the Pixel 3, Google is ditching the famous two-tone black-and-white "panda" color scheme for the device.
Before the remains were put on military planes bound for Hawaii, hundreds of USA and South Korean troops gathered at a hanger at the Osan base in South Korean for the repatriation ceremony, which included a silent tribute, a rifle salute and the playing of the US and South Korean national anthems and dirges in front of the United Nations flag-covered metal cases containing the remains.
North Korea's propaganda website, Uriminjokkiri, also criticized South Korea for its stance of keeping sanctions, saying "sanctions and conversation can not exist side by side".
Some 5,300 of those are believed to have been lost in North Korea, which is separated from the South by one of the world's most fortified borders.
A USA military aircraft left North Korea last week carrying 55 boxes handed over by Kim Jong Un's regime that are believed to contain the remains of US service members.
Buttressing the intelligence findings, independent missile experts this week also reported observing activity consistent with missile construction at the Sanumdong plant. At least one of the new missiles is another one of these.
"Pressure on the regime is clearly being felt", Pompeo said.
The remains were then moved in gray vans to an airfield where USA and South Korean soldiers loaded them one by one into two transport planes.
But the actual details of the process, including how and when the North's nuclear program is to be dismantled, have yet to be worked out. "We have not been taken for a ride", he said.
The signs of new activity at the factory came as generals from the rival Koreas met at their shared border for talks meant to ease a decades-long military standoff, according to Seoul officials.