An engineer on Friday said the seven blocks scorched in the Wang Fuk Court blaze could still be structurally safe even though some units had been gutted by the fire.

Edward Chan, past chairman of the structural division at Hong Kong Institution of Engineers, told an RTHK radio programme that concrete would start to crack and fall off at around 400 degrees Celsius while rebar would be affected when temperatures rise to 500 degrees.

When the mercury hits 1,000 degrees, he said, rebar begins to soften and lose its strength.

Workers from the Housing Bureau have begun reinforcement works at a small number of units that sustained more severe damage.

Chan said it was normal, based on what the Fire Services Department had said earlier, for an inferno to wreak severe damage to parts of a building.

"If the damage was especially severe on the lower floors, say all columns there were severely damaged, then it could compromise the building's safety," he said.

"But if the damage was localised, or found on higher floors, then it might not pose a major impact to the building's overall load bearing capacity."

The bureau, along with the Housing Department, will extract concrete core samples from the fire-engulfed buildings as soon as next week for a detailed structural analysis, with Chan saying more samples would have to be taken from units that suffered more severe damage.

By doing so, he said, officials will be able to find out whether the strength of the concrete structures still meets design standards or whether temporary support works have to be carried out if they do not, so as to ensure the buildings pose no immediate danger.