Wong King-man, deputy chief fire officer of New Territories North, said the vast area of the site, together with a large amount of building materials, had complicated the operation.

“The fire site is 70 by 80 metres [230 by 262 feet], meaning the size is more than 5,000 square metres,” Wong said. “There was lots of smoke and the temperature was very high, going beyond 500 degrees Celsius [932 Fahrenheit].”

The highly flammable oxyacetylene cylinders at the site also contributed to the intense fire, Wong said.

Firefighters were able to rein in the blaze after changing their defensive strategy to a more proactive one on Wednesday, authorities said.

Emergency personnel were unable to access the basement area of the site, where they said they believed the fire started, for more than a day.

Drones were deployed for aerial surveillance and robots sent to assist firefighters during the marathon bid to gain the upper hand.

Wong explained that the basement was like a maze, with metal and wooden frames strewn about that prevented firefighters from getting in.

Firefighters enter Hong Kong construction site to fight overnight blaze

As of Thursday morning, authorities had mobilised 741 firefighters and emergency personnel, 201 fire engines and 14 ambulances. Eight water jets and six breathing apparatus teams were involved.

At its peak, the fire was upgraded to a No 4 alarm. Fires in Hong Kong are rated on a scale of one to five, with five the most serious.

The Fire Services Department said earlier that explosions had occurred in the basement, possibly linked to the oxyacetylene cylinders.

The cause of the blaze is still under investigation. No casualties have been reported, but residents nearby were forced to close their windows to prevent the plumes of thick smoke from entering their flats.

Mary Chan Yuen-ming, a chief officer at the Buildings Department, said that two tower cranes at the site were under no immediate danger. Officers from the department would also stay behind at the site to provide assistance to firefighters.

She said further inspections were needed to determine when construction work at the site could resume.

“We need to go inside the site to do an assessment and to see if there is any follow-up work needed,” Chan said, adding that they would also inspect the supporting bases of the two tower cranes.

Hong Kong building where deadly fire erupted is warren of flats and businesses

The site will host 962 subsidised flats being built as part of the second phase of the Hung Shui Kiu/Ha Tsuen New Development Area under the Housing Society. Construction is expected to be completed in 2026.

The society said it would consider reinforcing two tower cranes at the site, or disassembling and replacing them, after the site reopened.

A separate fire broke out in a 60-year-old building in Kowloon on Wednesday, killing at least five people and injuring 43 others.