Hundreds of troops have gathered on Moheli, the smallest and least developed of Comoros' three islands, in preparation for the expected attack. Anjouan's self-declared leader is defying the central government by refusing to relinquish power.
Used to transport fuel for the military, the Tauris caught fire at 0200 on Sunday morning (2300 GMT on Saturday) in the port of Comoros' capital Moroni.
"An inquiry has been opened to determine the origin of the fire, whether it was accidental or criminal," government spokesman Abdourahim Said Bacar told Reuters on Sunday.
With a population of about 700,000 and lying off the east coast of Africa, Comoros has a long history of coups and inter-island frictions since independence from France in 1975.
Gathering steam for months, the latest political crisis has seen Comoros' federal government preparing its military to oust Anjouan's illegal leader, Mohamed Bacar.
First settled by Arab seafarers about 1,000 years ago, the tropical archipelago later became a haven for pirates.