Six people, including a firefighter, were killed and several others remained trapped more than 12 hours after a massive fire tore through a large shopping mall in Pakistan’s Karachi, officials said on Sunday.

Sindh Inspector General of Police Javed Alam Odho told reporters at the site that the blaze broke out at Gul Plaza, located on the city’s busy MA Jinnah Road, at around 10:45 pm on Saturday night.

As thick smoke engulfed the multi-storey building, rescue teams struggled through the night to reach those trapped inside. Sindh Chief Minister Murad Ali Shah directed Karachi Commissioner Hassan Naqvi to immediately investigate the cause of the fire and submit a detailed report, according to an official statement.

Rescue 1122 spokesperson Hassaanul Haseeb Khan said six fatalities had been confirmed so far, while around 20 people rescued from the building were taken to hospital for treatment.

Authorities said rescue operations were continuing amid fears that more people could still be trapped inside the gutted structure.

The firefighters and rescue teams were trying to put out the fire 16 hours after it started, officials said.

IG Aslam speculated that a short circuit might be the reason for the fire.

“Once the cooling starts, we will start further rescue operations, but the layout of the building is very complex as it houses a basement and mezzanine floor with hundreds of shops and stores,” rescue 1122's Khan said.

As many as 40 snorkels were being used and 60 per cent of the fire had been put out, he said, adding that the fire caused cracks in the mall’s walls.

Gul Plaza was built in the early 80s and was a popular destination for all classes as it had hundreds of shops for affordable crockery, decorations, furniture, electronics, clothes, women's cosmetics, perfumes, and other products.

A DawnNewsTV correspondent present at the site reported that all sides of the mall had been impacted, with one having “completely collapsed”.

The News International portal said firefighters stated that the lack of ventilation and the closure of windows at the mall — which spans over 1.75 acres, roughly the size of a football field — has led to smoke filling the interior of the building, hindering rescue efforts.

The shopping mall's building has become dilapidated due to the blaze's intensity. The ground and mezzanine floors have been completely burned, whereas the upper two floors are also on fire, it added.

Sindh Labour and Social Protection Minister Saeed Ghani told the media that there's still a possibility that many people are inside the building.

District South DC Khoso told Geo News, “There are fears that people are on the first and mezzanine floors. The rescue teams have not been able to reach these floors.” Prime Minister Shehbaz Sharif on Sunday expressed grief and said that possible steps should be taken to protect people's lives, The News International said.

President Asif Ali Zardari also expressed sorrow over the incident and conveyed condolences on the loss of lives.