North Korean officials were more taken aback than they let on after leader Kim Jong-un's summit with U.S. President Donald Trump in Hanoi collapsed, South Korea's spy agency told lawmakers.

Now that Kim has returned to Pyongyang, "it will take quite a while for them to review what happened to the summit," a National Intelligence Service officer was quoted by lawmakers as saying Tuesday.

But the NIS added Kim had made thorough preparations to prevent a coup in his absence, the longest since he took power.

"Before his departure, Kim had banned the use of airports and firearms, as well as suspending all military drills," a lawmaker said.