worsening/ setback

[vil]

Dear teachers,

Would you be kind enough to tell me which of the following sentences is more likely?

This is an unprecedented worsening in our relations.

This is an unprecedented setback in our relations.

Thanks for your efforts.

Regards,

V.

[chester_100]

Setback means impediment or hindrance. It doesn't necessarily mean a worse situation.
Worsening can mean aggravation; it refers to an already bad situation that even gets worse. It can be used as an adjective too.
I think they're both acceptable.