Donald McClean, Korea and the Bomb

["Martin Dunne" <martindunne@bigpond.com.au>]

In 1950 [McClean] became the head of the American Department in London and
passed to Moscow, among other things, the assurance obtained from President
Truman by Prime Minister Atlee that the United States would not use the atom
bomb in the Korean war.

p. 242 Phillip Knightley, A Hack's Progress, (Random House, Sydney, 1997)

This is just a throwaway line in Knightley's autobiography, but news to me.
Does anyone have any suggestions on further sources for this?

--
"We may agree that Jews were beaten, starved and sent to the gas chambers
because of the Nazi discourse of antisemitism, but it is quite a different
matter when someone tries to tell us that being beaten, starved to death or
gassed is also a 'discourse'."
- Richard J. Evans "In Defence of History"