staying with our world lead, another world lead, china telling the united states, back off. in a showdown at sea between two of the most powerful militaries in the world. beijing said it had tracked a u.s. navy warship that came very close to one of the artificial islands that china is constructing in the south china sea's disputed waters. the communist country issuing a stark warning to the u.s. that there could be consequences. national security correspondent jim sciutto joins us now. you've seen these bizarre manmade islands firsthand. >> i did. they're just mammoth feats of construction right in the middle of the south china sea. they're turning it into 2,000 acres of land over the past two years, equipping them with air strips, ports and what the u.s. military believes are military barracks, even artillery. all on seas the u.s. considers international and on piles of rocks the u.s. does not consider to be land. this transit by this u.s. destroyer intended to send that message. the voyage of the guided missile