~Everything will use AI at some point. This is like requiring a disclaimer for using Javascript back when it was introduced. It's unfortunate but I think ultimately a losing battle.
Plus if you want to mandate it, hidden markers (stenography) to verify which model generated the text so people can independently verify if articles were written by humans (emitted directly by the model) is probably the only feasible way. But its not like humans are impartial anyway already when writing news so I don't even see the point of that.
It would make sense to have a more general law about accountability for the contents of news. If news is significantly misleading or plagiarizing, it shouldn’t matter if it is due to the use of AI or not, the human editorship should be liable in either case.
This is a concept at least in some EU countries, that there has to always be one person responsible in terms of press law for what is being published.