Declaration of St. Petersburg, 1868
Hague Convention, 1907
Universal Declaration of Human Rights, 1948
Geneva Conventions, 1949
The Protocols Additional to the Geneva Conventions, 1977,
An of course, UN Security Council resolution 255 (1968) including the 1977 clarification is still supported by USA, UK, Russia which 'positive assurance' against the use of weapons against NNWS.
(Which as an aside makes Trump's recent tweet to end Iran illegal under US law and probably a war crime).
But of course, its precisely the posession of these weapons that makes all these niceties irrelevant.
Law of the jungle.
The real worldwide risk of nukes is direct confrontation between nuclear powers.
Mine doesn't have either.