Kolkata | A three-storey under-construction warehouse in the Taratala area of west Kolkata collapsed on Wednesday afternoon, trapping several people underneath the rubble, police said.

A Kolkata Police official said 13 people have so far been rescued. The warehouse was being built on a lease-hold property owned by the Syama Prasad Mookerjee Port in the city.

Of those injured, 10 were admitted to the Trauma Care Centre of the state-run SSKM hospital. Among them, four were stated to be in a critical state, hospital authorities said.

"The roof of the warehouse located on the Transport Depot Road near Brace Bridge in the Taratala area collapsed around noon. Our officers have reached the spot.

"Some people were working at the site at the time of the incident. We fear that quite a few people may still be trapped under the debris," the officer said.

Massive chunks of iron beams and concrete crumbled when several people were engaged in the construction work. Eyewitnesses said they heard trapped victims crying out for help from beneath the debris.

"Construction activities were taking place on the ground floor while the RCC structure for the first and second floors had been completed. Suddenly, the entire structure came crashing down," an eyewitness said.

According to a fire department official, the roof of the three-storey under-construction warehouse collapsed during casting work. He also alleged that substandard materials were being used in the construction of the warehouse.

Personnel of the Kolkata Police, Disaster Management Group, civil defence, and fire and emergency services teams were seen carrying out rescue operations at the site on a war footing.

Army officials were also seen aiding rescue operations, while cranes and heavy equipment lifting machinery were deployed at the site to remove the collapsed iron beams.

Gas cutters were being used to cut through the iron and concrete, and rescue workers were trying to reach the trapped victims by means of vertical drilling.

Sniffer dogs and overhead drones were deployed by the NDRF team to detect possible signs of life underneath the debris.

"We are following the cries for help coming from under the debris. At the same time, we are assuring the trapped people that they will be rescued soon," a member of the Kolkata Police disaster management group said.

Control room numbers -- 1070, 8697981070, 033-22143526/22535185 -- of the Disaster Management Group were opened at the state secretariat in the wake of the incident. Some locals alleged that large-scale illegal construction activities had been carried out in the area for some time. "We will definitely look into the causes of the accident and whether there were any irregularities. But right now, the priority is to rescue as many victims as possible," Minister Indranil Khan said. Officials of the Kolkata Municipal Corporation present at the site opined that there were prima facie evidence of design and construction flaws that may have led to the collapse. "It seems that the iron beams weren't strong enough to hold the weight of the overhead concrete. Also, I cannot see any braces which are required to support RCC casting. "We will have to check whether the design of the structure was approved by the civic body and, if it was, whether work was being carried out as per requirements of the approved design," a structural engineer present at the spot said. Urban Development Minister Agnimitra Paul and Kolkata Municipal Commissioner Smita Pandey have also rushed to the spot. Kolkata Police Commissioner Ajay Nand oversaw rescue operations at the site. BJP leader Rakesh Singh, who arrived at the spot earlier, claimed he feared multiple victims may have succumbed to injuries before they could be rescued.