SEOUL, March 20 (Reuters) - North Korean leader Kim Jong Un has overseen drills simulating a nuclear counterattack against the U. "Until now, North Korea has preferred mobile launchers for everything from SRBMs to even huge ICBMs, but given its poor road and system conditions, it was difficult to guarantee the stability of the missile during actual operations," said Yang Uk, a fellow at the Asan Institute for Policy Studies in Seoul. See here for a complete list of exchanges and delays