A horrific fire last week that killed dozens in a hulking Odessa building where pro-Russian protesters had taken cover was likely sparked by rebels on the roof who accidentally dropped Molotov cocktails, according to a preliminary investigation by the government.

The finding is likely to further anger separatists in eastern Ukraine who view the new government in Kiev with deep suspicion. It comes as authorities said the situation on the border with Russia has grown increasingly tense, and France warned of the prospect of...