Four men broke into a two-storey house in Bansdroni early on Wednesday, threatened the residents with a dagger and fled with cash and valuables worth a few lakhs, police said.

Proloy Biswas, an employee of the department of meteorology, said in his complaint with Regent Park police station that he woke up around 3.30am when some men entered their house by breaking down a ground-floor window grille.

While Biswas lives with his wife Babita and son Pushpak, a Class V student, on the ground floor, his brother lives with his wife Shukla and daughter Vasundhara, a Class VIII student, on the first floor.

The house at 80A Bansdroni Government Colony is a few hundred metres from the Ranikuthi crossing.

Biswas's brother who works in another city was not at home when the robbers struck, the police said.

One of the robbers pulled out a dagger and threatened to kill Biswas if they did not keep quiet. The robbers herded them upstairs and forced them into Biswas's brother's room where Shukla and Vasundhara were asleep, the police said.

The five were held hostage in the first-floor room while the gang went on a looting spree on both floors.

They broke open cupboards and took Rs 16,000 and gold ornaments and forced the women to part with their gold chains and bangles apart from a cell phone, Biswas said in his complaint.

"It is not clear whether the others were carrying arms. According to the statements of the family members, one of them had brandished a dagger," an officer of Regent Park police station said. "It seems other gang members were waiting outside."

Before leaving the house, the robbers told the family that they had planted a bomb in the house, which would go off if any of them tried to raise the alarm before 5am, the police said. It was after daybreak that the family stepped out and alerted their neighbours.