A blast in Kolkata's northern suburbs on Tuesday killed a seven-year-old boy and injured nine people, including the child's mother. The explosion occurred in front of a multi-storey building in the city's Nagerbazar area, police said. The cause has yet to be ascertained.

Deputy Commissioner of Police, Barrackpore Commissionerate, Ananda Roy said, "A child, who was severely injured in the blast, has died in hospital. A total of nine people are injured."

As per initial investigation, ammonium nitrate was used in carrying out the low-intensity blast outside the building, which also houses the office of South Dum Dum Municipality chairman Panchu Roy. "It was a socket bomb explosion. Forensic team and the bomb squad of CID are examining the spot. We have started further investigation," a police officer said.

Initially the police said that it was a high intensity blast.

"It was a high-intensity blast," an officer of the West Bengal Police said. "Four people were seriously injured, and six others sustained minor injuries. We found some iron nails (at the site), but we cannot ascertain the cause of the blast yet as there is no smell of gunpowder."

The explosion occurred around 9 am outside a fruit shop on the ground floor of the building in the congested neighbourhood, which falls under the jurisdiction of the Dum Dum Police Station. The injured were taken to the state-run RG Kar Medical College and Hospital.

A forensic team and sniffer dogs are at the spot, trying to ascertain the nature of the explosion, the police said. "Initially, we thought it was a gas cylinder blast, but it wasn't. We are trying to find out the details. Forensic experts, along with the CID bomb disposal squad, are examining the area," a senior officer of the Barrackpore City Police Commissionerate said.

The building where the blast took place also houses the office of South Dum Dum Municipality Chairman Panchu Roy, who claimed to be the actual target of the explosion.

Stopping short of naming any political party, Roy said the forces currently attacking the ruling Trinamool Congress across West Bengal were behind the attack. "It was a planned blast... They had planned to kill me and other TMC workers, as it would create panic and help them gain foothold in the area," he was quoted as saying by PTI.

Asked whether the explosion was a result of infighting within the TMC, Roy said there was no such issue in South Bengal.