They can already hit Israel with their IRBM's.
And perhaps they could shoot a small nuke missle at a large fleet of U.S. warships.
Hitting a constantly moving target with a ballistic missile is not the easiest thing to do. Besides, the SM-2 and now the SM-3 missile can most likely shoot it down.
An airburst above a fleet of ships wouldn't do the ships much good. Close counts with nukes. Don't have to hit anything dead center.