Fear for

Grammatically you need to say either "reprisal attacks" or "a reprisal attack", but "fearing for" is not right here either. "fearing for something" usually means that the "something" is desirable (or at least neutral) and you fear that you will lose it or it will suffer a bad outcome (for example, "fearing for one's job", "fearing for the future").

You could say:

Thanks Mr Wordy. What about this sentence?

They are fearing that the army may/will mount a reprisal attack.

Abil
They are fearing that the army may/will mount a reprisal attack.

It's OK, but "They are fearing..." is less likely than "They fear..." (This applies to your original sentence too; sorry, I forgot to mention it.) One would need to know the full context to be sure.

Mr WordyOne would need to know the full context to be sure

The context is: tensions had been brewing for the last few days between two ethnic communities - say X and Y -- over the ownership of a piece of land, and after an initial clash one of these two communities, X, is gripped by the fear that the people of Y community might launch further attack on th

Despite the sizable number of Google hits, "They are fearing for the worst" is in my view not correct. You should say "They are fearing the worst" or "They fear the worst".

Sorry, by "context" I meant the exact words used in the surrounding text. Several examples have been given ("They are fearing reprisal attacks", "They are fearing the worst", "gripped by fear") and I'm not sure