The South Korean news agency Yonhap reported Tuesday that Kim - who dropped out of the public eye from late July to mid-August - inspected troops near South Korea's outpost in Yeoncheon, which is close to the 2.5-mile-wide demilitarized zone that divides the warring countries.
The United States and South Korea are watching North Korea carefully for any signs of provocations as USA and South Korean militaries began joint war games on Monday that are held every summer and always infuriate Pyongyang.
Trump appeared to refer to threats of "fire and fury" if North Korea continued its provocative behavior toward the United States and its allies.
"I am pleased to see that the regime in Pyongyang has certainly demonstrated some level of restraint that we have not seen in the past", Mr Tillerson told reporters at the State Department.
Using solid rather than liquid fuel would allow North Korea to launch missiles with less preparation.
The production will be increased by "further expanding engine production process and the production capacity of rocket warhead tips and engine jets by carbon and carbon compound material", at the institute, KCNA reported.
"The research institute has also succeeded in researching, developing and localizing 3D carbon/carbon - Silicon carbide composite, which are the most advanced materials used for producing warhead tips and solid-fuel engine exhaust nozzles for intercontinental ballistic rockets", KCNA added in Korean.
North Korea released a chilling propaganda video Tuesday showing President Donald Trump standing in a graveyard and Vice President Mike Pence in flames as the United States and South Korea prepare for joint military maneuvers.
The same exercises previous year provoked North Korea to conduct nuclear tests.
The manufacturing process included carbon fibre weaving, chemical deposition and high pressure liquid deposition, KCNA said.
He said earlier this month that he would watch the US' conduct "a little more" before deciding whether to fire ballistic missiles towards Guam, a U.S. territory in the Pacific, as he had threatened.
The diagrams on the wall behind him showed a missile called Pukguksong-3 as well as one that appeared to be in the Hwasong series, according to The Associated Press.
He also expressed hope that it was a sign of Pyongyang's readiness to enter peace and disarmament talks with Washington "sometime in the near future".