If countries that have nuclear weapons wish to give an assurance to countries that do not have nuclear weapons that they will not use nuclear weapons against them, then obviously it is open to those countries to give those assurances unilaterally.
如果拥有核武器的国家希望向无核武器国家保证不对这些国家使用核武器，那么显然它们尽可单方面对无核国作出保证。