No the US are not in any threat as their missiles are not strong enough to even come close. It would be absolute suicide to attempt something without Chinas approval which they currently do not have. China is a superpower hugely due to the Western world and although the are powerful as powerful gets, I do not believe for a second they would try and take out their main sources of trade, even with all the money they currently have. From their appeal for calm today, it seems that they clearly disagree with NKs threats. Ignoring China would be poisionous for their relationship without just cause (I mean this is all over not being trusted with nuclear weapons mainly - yeah NK, we'll totally trust you now!!). The re-opening of the Nuclear facility in Yongbyon seems threatening but I question how advanced their research really is and can get to in such a short breadth of time.
What we are in danger of is N Korea attacking S Korea as they did a couple of years ago (they sunk a warship and also there were additional fatalities). That in itself would cause the US to step in more severely as they have vowed to protect the South already. We have to remember this man, has been fed propaganda and lies all of his life by his father and advisors - I have no doubt that he believes he is more powerful than in reality. He is in a room full of yes men and I'm not really quite sure what will stop him (although more aid is probably the key factor). But then again how much can we justify given more aid to a country that wont feed it's own people, preferring glitzy parades, building a sheltered tourist industry.
By the way there is NO WAY they can reach the UK either. Not that we're currently involved really. I'd be absolutely gobsmacked if China keeps North Korea as an ally, many would disagree with me but China, although communist technically, is not true communism ('Maoism' instead of 'Marxism' - NK is comparably Stalinist). I don't see the relationship being sustained purely on political ideologies over economic growth. Perhaps the delay in China giving a firm answer on who they will ally with is in fear of NK throwing a missile in their direction as a kind of Dr Strangelove moment - I hope the silence means they're all secretly planning a disarmament tactic together - the US have just planted a missile defence system on the border so maybe? Dunno how plausible that is though. Eeep!
***I made a couple of mistakes when I first wrote this because I was half asleep and didn't double check my facts, edited now !