More than 60 residents evacuated or fled their homes after a fire broke out in a second-floor unit at Block 147, Serangoon North Avenue 1 at around 7.10pm yesterday.
Two women who later complained of breathlessness were taken to hospital.
Mr Kenddrick Chan, 26, an undergraduate, saw the fire break out at about 7.10pm from his 11th-storey unit at neighbouring Block 146. He said there was a "strong smell of burning rubber as white smoke quickly turned to grey and engulfed the second floor and above".
Mr Chan shouted at onlookers in the units above to close their windows. He then heard a loud "pop" sound as the whole block went dark. "They seemed to be in a daze. A crowd below was taking pictures, but those in the block were just staring and pointing at the fire. So I quickly called 995," he said.
He said he saw a man on the second floor, who looked dazed, being led down by firemen using a ladder.
Ms Letchimi Subramaniam, 56, a caregiver in a nursing home, said she heard screams at around 7.15pm from her fourth-floor unit at Block 147. She rushed out and joined a crowd of onlookers at the foot of the block.
The Singapore Civil Defence Force (SCDF) said it was alerted to the fire at about 7.15pm. The contents of a bedroom had been on fire. The fire was extinguished using compressed air foam and one water jet.
SCDF and police officers evacuated 16 residents from the second to fifth storeys, whose units were in close proximity and directly above the affected unit. Another 50 residents left their homes on their own.
A woman in her 80s and another in her 30s were taken by ambulance to Tan Tock Seng Hospital after suffering from breathlessness. The cause of the fire is being probed.
Ms Sylvia Lim, Workers' Party chairman and Aljunied GRC MP, was on the scene at around 9pm. The block is in her ward.
She said: "The incident was an illustration that fire safety can never be taken for granted. Our first priority is the safety of our residents. We are working to restore power in the block as fast as we can."