The word "could" means it is possible , the word confirmed means it surely happened.
Big difference.
In none of the article above there is a mention of "15 rebel tanks" you also claimed.
Furthermore:
Reuters quoted a military source saying that as many as a thousand of fighters were engaged in fierce fighting around Krasny Liman, involving armoured vehicles, possibly tanks.
Also , again you are messing with the news:
The paragraph you quoted is followed by another statement:
Up to 4,000 separatist fighters could be involved in Thursday's fighting near Krasny Liman, and armored vehicles and possibly tanks were being used by both sides, the military source said.
The reported use of tanks could not be independently confirmed.
So , you're spreading false news and claims again... typical