It said North Korea had installed the environmental shelter in late 2015 to hide detection of test preparations.
Missile experts are waiting for higher-resolution photos of the rocket engine from North Korea's state media for clues about the technology.
In South Korea on Friday, Tillerson said a United States military response would be on the table if North Korea threatened South Korea or USA forces.
In an English dispatch by the Korean Central News Agency (KCNA), the North said the test was meant to "confirm the overall technical indices of the engine such as features of thrust power in the combustion chamber, accurate movement of turbine pump, control system" and "structural safety and reliability" of the tested engine.
The UN believes North Korea is using its so-called space programme to design a missile capable of hitting the US.
Seoul has blamed Pyongyang for his death, but the North has rejected those claims and denounced Malaysia's investigation as an attempt to smear the secretive regime, insisting that Kim most likely died of a heart attack.
NCAA tournament: UC Davis' Chima Moneke spans globe
So I hope he can get here. "We have a chip on our shoulder, individually and collectively, " said senior guard Darius Graham. He plays a lot of 4 for the Aggies, so he will be an interesting cover for the Jayhawks, even with their small ball lineups.
The test occurred as Secretary of State Rex Tillerson was due to meet with Chinese President Xi Jinping in Beijing.
Kinzinger - a veteran who has served in Iraq and Afghanistan - said he takes Kim Jong Un seriously "because I think he's nuts" and believes the answer to a de-escalation of the North Korean threat lies with China and Russian Federation.
"If the North Koreans refuse to help, we will look at other laws which can be used to arrest the four suspects".
Its smaller size could mean that the rocket was built for a smaller missile - or for the second stage of a bigger missile, like an ICBM, she said.
The term "Juche" refers to North Korea's homegrown ideology, or unique way of thought.
North Korea signed the Nuclear Non-Proliferation Treaty in 1985 and then after multiple violations, formally withdrew 18 years later. While Mr Tillerson has said a key component of his new effort involves pressing China - North Korea's main ally and economic lifeline - to better enforce United Nations sanctions, Mr Wang said the matter is primarily between the USA and Pyongyang.