Debris left behind by contractors after renovation works could be the reason why an HDB lift malfunctioned and resulted in several residents being trapped inside.

Mr Liu Yu Zhu, 33, who lives in Block 511A, Yishun Street 51, was leaving for work about a week ago when the lift got jammed two floors down from his seventh-floor flat.

He was stuck inside for 20 minutes before another resident managed to help him pull the doors open and get him out.

Nee Soon Town Council (NSTC) said there were two reports filed about the same lift - on June 21 and June 22- getting jammed, though on one of the occasions, the lift was empty.

An NSTC spokesman said the stoppages were likely due to debris or objects getting stuck in the gap separating the lift carriage and the walls.

WORRIED FOR CHILDREN This is a new block and it's not fully occupied. What if they shout for help, and there is no one to help them? RESIDENT DENNIS ANG, a father of two young boys, who moved into a fifth-floor flat last month.

The Straits Times understands that several flats in Mr Liu's block are undergoing renovation. Keys were given to residents in April.

This is the third incident in recent weeks where contractors have been linked to issues in new housing blocks. On Sunday, The Straits Times reported how a 61-year-old woman twice found her home in Punggol flooded by sewage, prompting the HDB to advise contractors not to put unwanted materials or debris into waste pipes.

Meanwhile, it was reported last week how rubbish chutes from the second to eighth floors at Block 817A Keat Hong Link were stuffed with litter. The HDB found that refuse handling equipment attached to the central chute on the ground floor had malfunctioned.

An HDB spokesman said the problem was caused by the improper disposal of bulky refuse and renovation debris, which caused the chute to get jammed.

Mr Liu, who works in a meat processing factory, said: "I hope they repair the lift, so that people won't get trapped in it again."

Fellow resident Dennis Ang, 43, said the lift at the block malfunctioned on June 21. He saw about 10 people gathered at the lift landing on the ground floor at about 7pm.

Two seniors and two young children seemed to be trapped inside and a few people were trying to pull the doors open, but failed.

Mr Ang said an elderly woman trapped in the lift was "quite hysterical", pulling at the door and shouting "Let us out" in Mandarin.

Mr Ang went to get help from the management office and four of them were freed about 15 minutes later. "This is a new block and it's not fully occupied," said Mr Ang, a father of two young boys who moved into a fifth-floor flat last month.

He said he is worried his children might get trapped in the lift. "What if they shout for help, and there is no one to help them? " he said.

On Thursday last week, he noticed that the digital information display at the lift said it was overloaded, when it was, in fact, empty.