The Journal ran a piece on the NORK missile program today. They showed sketches of three NORK missiles, all liquid fuel. Later on in the article they mentioned that the NORKs were working on, and had test fired at least one solid fuel missile. But the missiles in the sketches were all liquid fuel.
Why do we care? Liquid fuel is half liquid oxygen (LOX) and the other half is something like kerosene, or alcohol, or perhaps even hydrogen. As soon as the missile is fueled with LOX, the LOX starts boiling off. The boiling point of LOX is minus 183 degrees Centigrade. Putting LOX inside an aluminum missile fuel tank in ordinary air (plus 20 C) and the LOX starts to boil off into oxygen gas. The fuel tank pressure relief valve lets the gas vent outside to keep the tank pressure down. Not a problem if you launch right after fueling, but you cannot keep a liquid fueled missile fueled for more than tens of hours. Give it ten or twenty hours and all the LOX is gone, boiled away. You cannot keep a fleet of liquid fueled missiles fueled and ready to launch at a moment's notice. After the order to launch is given, figure an hour or two for fueling before main stage ignition.
Far better as a weapon is a solid fueled missile. The earliest solid fuel was plain old black gunpowder, and 4th of July skyrockets still use gunpowder. Far better solid fuels can be made but the chemistry is complicated. Modern solid fuel has a lot of plastic explosive in it, usually some powdered aluminum for extra hot burning, and a lot of magic chemicals to slow the burn rate down so it doesn't just explode and blow the missile to bits. Just what the magic chemicals are is secret, and tricky. We didn't get it figured out until the first submarine launched Polaris missiles in the 1960's. It will take the NORKs a while to come up to speed on making solid fuel missiles.
With solid fuel missiles, you can launch within a minute or two after the red phone rings with a launch order. Which makes a much more reliable weapon than liquid fuel missiles.