AKICIF: British Capitalization of Acronyms for Organizations

(Or "British Capitalisation of Acronyms for Organisations", if you prefer)
The Guardian and the BBC seem to write the abbreviation for "Immigration and Customs Enforcement" as "Ice", rather than the "ICE" used in the US.   But they abbreviate "United States" as "US", not "Us".
What's the rule here?  Is "North American Treaty Organization" written as "Nato"?
--
Evelyn C. Leeper, http://leepers.us/evelyn
"When I was a child, I spake as a child, I understood as a
child, I thought as a child: and when I was a man I decided
how foolish it would be to give all that up." --Mark R. Leeper