The fire had been extinguished and efforts to find more possible victims inside the building continued, Susatyo Purnomo Condro, the head of Central Jakarta police, told reporters on Tuesday.
The fire erupted on the first floor about midday, Condro said, and then spread to the upper floors.
Some of the employees were having lunch in the building at the time while others had left the office, he said.
As of Tuesday afternoon, the death toll had reached 20, he said.
"Now, we are still focusing on evacuating victims and to do fire cooling," Condro said.
The building is the office of Terra Drone Indonesia, which provides drones for aerial survey activities with clients in the mining to agriculture sectors.
The company is the Indonesian unit of Japanese drone firm Terra Drone Corporation, according to the company's website.
Footage broadcast by Kompas TV showed dozens of firefighters were trying to evacuate the people inside, and some were carrying body bags from the building.
Some workers were also seen escaping from the high floors of the building using portable ladders.
Both companies did not immediately respond to requests for comment via email.
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