The following from Lutkins1 (Acting Director Chinese Affairs) on 20th August:
4. Lutkins commented in same terms as Marshall Green regarding the State Department view that the Russians would be reluctant to make available nuclear weapons to the Communist Chinese for fear of being dragged into nuclear war through rash Chinese Communist adventures. Lutkins thought that the Russians were extremely sensitive about extending 'nuclear club'. At any rate this was the impression which Soviet officials had been seeking to give to the United States representatives for some time past.
[NAA: A1838, TS695/5/5]