"We see more people crushed under the debris," Salam told The Associated Press from the site of the accident in Dhaka's central Tejgaon district. "So the death toll is likely to go up."
Twenty bodies had been recovered, with army troops joining the rescue work, he said.
Salam said the four-story building was built on what was once a canal and the owner had been adding another story to it. Local media said the owner had not sought permission from the city for the additional floor. They cited officials from the Capital Development Authority, which oversees building construction in Dhaka.
Another fire official, Ataur Rahman, said it was not known how many people were inside the building at the time of the accident. The Cabinet minister in charge of overseeing local governments, Jahangir Kabir Nanak, said the government ordered an investigation into the collapse.
Dhaka is a teeming city of more than 10 million people, and its many of the buildings are constructed without proper design and with poor materials.