Not sure exactly what the proof is required for me to believe it, but a single article on reuters doesn't do it. (Interestingly enough, AFP also reported the Mosul incident in a similar manner to the recent incident--very skeptical of US statements, but not of the Iraqi).
Even stories that are picked up by many news agencies are just plain wrong. The widespread looting of the museum in Baghdad, for example, falls into this category. Everybody was reporting it, but in the end everybody seemed to be duped. Beyond that, sometimes the "ground level" view is not in context and appears to be something that it isn't.
Highly politically charged issues like this are extra suspect, because of several factors:
1) There are people who have a vested interest in presenting the US in as bad a light as possible without any scruples at all
2) There are people making eye witness reports that are pissed off (for example these Iraqis being insulted by being occupied)
3) And there are biased reporters who let (intentionally or not) their personal animosity against the US or its current administration bleed through--either by slanting the reporting, or simply by not fact checking because they'd rather believe what they're reporting.
4) There's just stupid reporters out there.
Of course, reversing the political leanings of the list above also leads to the same thing.
I think the story needs widespread coverage and legs, or ongoing pattern, or widespread eyewitness reporiting before I'll be satisfied with it being a somewhat accurate reporting of the situation.