Well, the nukes probably are not sitting at a known launch site. There are probably several within short range missile distance of South Korean targets.
The US, being the US, would probably cheerfully be attacking North Korea at this moment if it were not for the very real possibility that its allie in the region, South Korea, does not want to raise the very real risk of being nuked. This is the problem with GW Bush's declaration that NKorea was part of the evil axis a decade or so ago... he made it clear that NK was subject to attack.