"This is one of North Korea"s major nuclear modernisation goals," said Ankit Panda, a nuclear policy specialist at the Carnegie Endowment for International Peace."We should expect to see the first flight tests of longer-range solid fuel missiles in the coming months," he told the BBC.He added the expansion of North Korea"s ICBM arsenal presented potentially "serious military planning challenges" for the US in a conflict, as it would be difficult for the US to find and destroy all of North Korea"s mobile ICBMs.