The death toll from a boiler explosion and subsequent fire at a packaging factory in Bangladesh over the weekend has risen to 31 after more bodies were found in the debris, an official said.

The huge blast early on Saturday at the Tampaco Foils factory outside the capital, Dhaka, triggered a massive blaze that spread quickly at the plant, where flammable chemicals were stored.

Twenty-three workers died immediately. Six more bodies were recovered on Sunday evening, and two others were found overnight as firefighters cleared the debris, said local chief administrator SA Alam.

At least eight workers were still missing on Monday, Alam said, adding that the search for bodies would continue while authorities try to determine what caused the explosion.

READ MORE: Bangladesh factory engulfed by fire, 24 dead

Saturday was the last working day at the factory before employees were to go on leave for a weeklong holiday for the Muslim holiday of Eid al-Adha, which falls on Tuesday.

The workers who died were nearing the end of their overnight shift when the explosion took place at around 6am.

Thirteen people are being treated for burn injuries at the Dhaka Medical College and Hospital, including six in critical condition, Mohammed Bacchu Mia, a police officer at the hospital, said.

Anxious relatives crowded the hospital on Monday looking for their family members.

Rajesh Babu, who worked as a cleaner at the factory, has not been seen since Saturday, said his mother, Mina Rani Dey.

"He came to work early in the morning on Saturday. He has not returned," Dey said, sobbing as she held a photograph of her son. "His father has become sick because our son has not returned."

Factory safety is a major concern in Bangladesh, which has thousands of garment and packaging factories that supply products to global chains like Wal-Mart and H&M.

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A fire at a garment factory in a Dhaka suburb killed 112 workers in 2012. In 2013, a commercial complex near Dhaka housing five garment factories collapsed, killing 1,135 people, Bangladesh's worst industrial disaster.

Those two disasters prompted Bangladesh's government, global brands and the United Nations to work together to try to improve safety standards in the South Asian country's factories.

Source: Agencies