It's coercion... do as I say or else. That's illegal and certainly against any 'norms'.
It's not a different approach to the use of force - it's simply a deferred use of force.
If this were deemed legal then the precedent could be used to coerce nations to sign any treaty and probably trade agreements they oppose as well if the treaties and agreements were recognized as being 'the international norm'. A very lucrative prospect, but I digress.
Even if it might work it's still illegal. It's really sad that no-one even seems to consider acting within international anymore while upholding 'international norms'. Anyway, if it could stop the impending disaster and nobody is going to respect international law anyway I'm all for it. But one should keep in mind that going down this route will produce fallout in the future by setting a precedent for state-sanctioned blackmail. As long as you keep that in mind and work real hard to reduce the unintended consequences... 
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