When there is a possibility that it was committed by your ally.
“We do express our sympathies to the victims and their loved ones who died in this horrific explosion,” Miller [US State Department] said.
https://www.aljazeera.com/news/2024/1/3/heinous-world-reacts-to-twin-blasts-that-kill-dozens-in-iran
They can’t utter the words “terrorist attack”. Just in case.