This is the eerie moment hundreds of black birds swarm over city streets and fly around in circles.

The ominous sight was filmed by residents over the sky of Ulanhot, in northern China's Inner Mongolia region, on the evening of October 20.

Phone footage shows what looks to be hundreds of birds densely packed in a flock and flying at a low altitude over the street.

Dozens more gathered on top of a near building and all over treetops as the rest circled around street lights.

Birdwatchers described the birds emitting beautiful and haunting chirps as they circled low in the sky, forming a massive cloud of black feathers. The event sparked widespread speculation and curiosity among locals, with many users fearing it might be a bad omen.

One said: 'Where the crows fly, people will die.'

Another added: 'Mankind continues to be bad, and calamities will be inevitable.'

Experts said the phenomenon might be a migrational behaviour of a particular bird species while heading south for warmer climbs.

Officials urged the public to remain calm, reminded everyone that it is a part of bird migration, and advised caution to prevent unnecessary traffic congestion or safety hazards during observation.