> ... seek immediate UN Security Council action to provide assistance to Ukraine ... if they should become a victim of an act of aggression or an object of a threat of aggression in which nuclear weapons are used
This seems like the passage which would cover it. The UN is able to authorize use of force by member states against the aggressor. Though it looks like it hasn't done that - probably because of Russia's permanent position on the UN Security Council which would veto any such measures.
> ... seek immediate UN Security Council action to provide assistance to Ukraine ... if they should become a victim of an act of aggression or an object of a threat of aggression in which nuclear weapons are used
This seems like the passage which would cover it. The UN is able to authorize use of force by member states against the aggressor. Though it looks like it hasn't done that - probably because of Russia's permanent position on the UN Security Council which would veto any such measures.