“Les États-Unis” is a great example of liaison in French: there are two strong liaisons in “z”, one after “les” (or “des” or “aux”…) and the other one after the “s” of “états” (note that the t is silent though).
“Les États-Unis” is pronounced [lé zéta zuni]
“Des États-Unis” is pronounced [dé zéta zuni]
“Aux États-Unis” is pronounced [o zéta zuni]