A mystery black smoke ring baffled locals as it floated through the sky in Thailand.

Footage shows the dark hoop of smoke drifting over a neighbourhood in Nakhon Pathom province on October 27.

Confused residents stared at the bizarre sight until it dissipated.

Ajarn Jessada Denduangboriphan, a lecturer from Chulalongkorn University's Department of Biology, said the phenomenon has also been seen in other countries, and was not a swarm of insects flying through the air.

He said it was believed the black ring may have been caused by 'the explosion of a spherical structure', with the smoke maintaining its ring shape as it rose.

He said: 'It may have been caused by a transformer explosion. The source of the smoke should be clearly investigated. If the cause was a machine or burning fuel, the smoke will just fade into the air. But it might be harmful if it was caused by the burning of certain chemicals.'

Another black smoke ring caught tourists' attention when it appeared in the sky over Sentosa in Singapore in August. A spokesman for the Mount Faber Leisure Group later clarified the ring was caused during the testing of flame generators for a lights and fire show on the island.

While in March, Russian citizens spotted a similar dark halo floating over buildings in Moscow. Local authorities have not commented on the incident.