Retired contractor Daniel Raj was at home in his eighth-storey unit in Block 538 Hougang Street 52 on Wednesday afternoon when he heard "at least two blasts".
He said one of these was a big one that "sounded like a gas drum exploding", and that from his flat, he could see a second-storey unit on fire in the adjacent Block 537.
"The fire was bursting out of the kitchen window... I could hear what sounded like gas blowing out to feed the flames," added the 69-year-old.
The Singapore Civil Defence Force (SCDF) said on Facebook that it was alerted to the fire around 5.45pm. The blaze, which was put out by firefighters, was linked to the contents of the kitchen.
Five people - said to be family members from a seventh-storey unit at Block 537 - were taken to the Singapore General Hospital and the National University Hospital for smoke inhalation.
No one else was hurt.
Retired technician Johari Sa'at, who lives next to the unit that caught fire, told The Straits Times yesterday: "A lot of people live there... I see different young people, (I) don't know if they are renting."
Another second-storey resident, who wanted to be known only as Ms Xie, told Shin Min Daily News on Wednesday that five people were living in the flat, and she noticed that they regularly received visitors.
The 45-year-old clerk said: "I had been worried that with so many people in the unit, clutter would build up - but little did I think something bad would actually happen."
Mr Johari, 68, who was shopping for groceries when the fire broke out, rushed home after receiving a call from his wife about the incident. By the time he got back, she had "run downstairs already". He went upstairs to make sure everything in their flat was turned off before joining her.
About 20 residents from the upper-floor units of the block evacuated their homes before the SCDF arrived. Mr Johari said he saw two women receiving oxygen from paramedics.
When ST visited the block around 4.30pm yesterday, an acrid smell hung in the air. Soot could be seen on the external walls of units up to two levels directly above the burnt flat.
The ceiling at the staircase landing outside the affected unit was also covered in soot.
Beneath that flat is a unit housing the Ang Mo Kio-Hougang Angsana Residents' Network. Maintenance staff could be seen scraping the ceiling and sweeping up debris on the floor.
When approached, an unidentified staff member declined to comment, except to say that the work was unrelated to the fire.
Mr Darryl David, an MP for Ang Mo Kio GRC and chairman of Ang Mo Kio Town Council, told ST that "the common area has been cleaned up and we have made arrangements to repaint the affected ceiling". He added that the damaged electrical cables and lights in the common area have been replaced.
"We will continue to render necessary assistance to the (people in the) affected unit and coordinate with other agencies where necessary," he said, adding that SCDF is investigating the cause of the fire.
