Firefighters say some of the fire doors at a Jordan building where a blaze killed five people on Wednesday were faulty, and this may have led to heavy smoke in one of the staircases.

Speaking on an RTHK programme, a divisional commander from the Fire Services Department, Lam Kin-kwan, said nothing was amiss when they last inspected New Lucky House on February 9.

But during Wednesday's rescue operation, officers did find hazards, Lam said.

"There are three staircases. One of them was basically smoke-free, and more than 100 people used that staircase to leave the building," he said.

"The most badly affected one was the number one staircase at the main entrance. After the fire, we found out that some fire doors were broken, which may be one of the reasons there was heavy smoke."

Lam added that the fire probably started in the lobby or lightwell, which were close to the smoke-filled staircase.

The law stipulates that building owners must have fire service installations inspected by a registered contractor at least once a year, but the fire officer said evidence suggests this rule was not complied with at New Lucky House.

He said an inter-departmental task force will look into the cause of the fire and why there were so many casualties.

Lam said they will check whether illegal changes had been made to the layout of the building, which would have added to the difficulty in the rescue operation.