I don't believe there's any solid references to the UN in any of the books (could be in Armies of the Night though).
Personally, once the nukes started, and definitely after November 1997, I tend to think the UN is little more than an impotent shell - still some officials floating about trying to puff up their importance, but nobody takes any notice of them while they're focused on putting food on the table and shelter over their heads.
The UN offices in New York are probably destroyed, certainly not in use anyway except as perhaps shelter for the diehards.