URN for ECDSA signature algorithms (including hash algorithm)
According to the IANA registry at https://www.iana.org/assignments/urn-namespaces/urn-namespaces.xhtml, there is no nist-gov URN namespace.  Outside obscure antisocial protocols like RFC 2802 there is no reason to try to use this URN namespace.  You appear to have gotten these URNs from a defunct internet-draft.  If you're using XML signatures, as you appear to be doing, then you can use use the URI http://www.w3.org/2001/04/xmldsig-more#ecdsa-sha256 like RFC 4051 specifies.

(But consider throwing XML signatures in the garbage—if not XML too.  Just sign binary blobs with a fixed signature scheme baked into your application; using XML signatures is asking for trouble.)