North Korea's November ICBM "has not yet shown to be a capable threat against us right now. We are still examining the forensics, we're still doing the forensics analysis," CNN quoted Mattis as saying during an off-camera briefing with reporters at the Pentagon on Friday.
Mattis' comments came weeks after North Korea tested what it said was a new kind of ICBM capable of reaching the entire US mainland. At that time, he had said that the missile flew higher than any previous test by the secluded country.
However, his assessment on Friday was in sync with technical analysts who opined that North Korea's November launch did not actually demonstrate an ability to hit the US.
"As I said earlier this week, a sustained cessation of North Korea's threatening behaviour must occur before talks can begin," Tillerson told the UN meeting, convened to discuss Pyongyang's denuclearisation, adding "North Korea must earn its way back to the table."
Pyongyang has taken its weapons tests in recent months a notch higher.
In September, North Korea tested what it said was a hydrogen bomb.