A portion of an under-construction flyover collapsed today in the densely populated BarraBazar area in Kolkata, West Bengal, India.

The fatal incident has caused reportedly caused the death of at least 17 people and has left more than 100 injured.

According to the Kolkata Police, about 30 people have been taken to hospitals, while many more victims are yet to be rescued, reported The Indian Express.

"Use of sub-standard materials and lack of proper inspections led to the collapse of the flyover."

Rescue operations at the site are ongoing and the Indian Army and teams of the NDRF have been deployed into service.

Hindustan Times reported that the rescue workers used heavy drilling machines to cut through stone slabs in order to save lives of the victims trapped under the debris.

Paramilitary forces that were deployed in the city on election duty have arrived at the site to help the rescue workers in their efforts, while local residents also jumped into rescue operations.

Indian Prime Minister Narendra Modi was quoted by The Indian Express as saying: "Shocked, saddened by collapse of under construction flyover. Took stock of the situation and rescue operations."

Having been under construction since 2009, the flyover has missed several deadlines for completion, reported Reuters.

According to experts, use of sub-standard materials and lack of proper inspections led to the collapse of the flyover, reported BBC News.

Several cars, trucks, nearby buildings and passers-by have been struck by the collapse.

The narrow lanes, and closely built shops and houses in the area limit the access for heavy lifting equipment to the area, thereby making rescue operations difficult.