At least 13 people have been killed in an explosion at a firecracker factory in India's southern state of Kerala, officials say.

The incident took place in Thrissur, where around 40 people were believed to be assembling firecrackers in a cluster of sheds for an upcoming festival. Several others were injured, with five reported to be in critical condition.

This was the second such incident this week. On Sunday, a blast at a firecracker factory in neighbouring Tamil Nadu state left at least 25 dead.

Accidental explosions are a grim routine in India's fireworks industry, recurring with troubling regularity - most visibly around Sivakasi in Tamil Nadu, which produces much of the country's supply.

The explosion in Thrissur on Tuesday around 15:30 local time (10:00 GMT) triggered secondary blasts that slowed rescue efforts.

Firecrackers were being made in temporary sheds near paddy fields with poor road access, hampering emergency response.

The force of the blast was felt kilometres away - some residents mistook it for an earthquake - and nearby homes were damaged.

Eyewitnesses described chaotic scenes as locals joined rescue efforts, pulling out the injured and dead.

Kerala's Revenue Minister K Rajan told the BBC that all fireworks units preparing for the upcoming Thrissur Pooram - a major annual Hindu temple festival - had permission to manufacture and store fireworks in designated areas.

The workers were preparing fireworks for the Thiruvambady Temple, one of the two main participants in the climactic displays of the Thrissur Pooram.

The festival's famed fireworks - staged by Thiruvambady and Paramekkavu Bhagavathy Temple - unfold in four phases, with the main overnight display drawing huge crowds on the seventh day.

Officials said food had been prepared for about 40 workers, suggesting at least that many were present when an explosion hit a shed.

While municipal chairman PN Surendran attributed the blast to high afternoon temperatures, officials said the cause remains unclear.

"There is still no clarity on how many workers were in the shed or the extent of injuries. It is suspected that extreme heat may have contributed to this tragic incident," Surendran told reporters.

K Gireesh Kumar, secretary of the Tiruvambady temple committee, said the group had permission to store 2,000kg of firecrackers for each display.

A magisterial inquiry has been ordered to establish what happened and fix responsibility.

Kerala Chief Minister Pinarayi Vijayan has said that medical experts from other states would be brought to treat the burn victims, if needed.

Prime Minister Narendra Modi has announced financial assistance of 200,000 rupees ($2,137; £1,582) for the families of the deceased persons, and 50,000 rupees for those injured.

Firecracker factory accidents are common in India because highly volatile chemicals are handled in cramped, low-cost setups where even a small spark can trigger explosions.

The industry - which is largely informal - supplies firecrackers for weddings, festivals and other ceremonies and has faced repeated scrutiny over safety standards.

Around hubs like Sivakasi in Tamil Nadu, weak enforcement and festival demand during Diwali often push safety to the margins.

One study recorded 9,896 accidents in the sector in Sivakasi between 2003 and 2010 alone, including 398 fatal incidents.

In Kerala, a 2016 explosion and fire at the Puttingal Temple in Kollam district - where stockpiled firecrackers ignited during an unauthorised display - killed more than 100 people and injured 400 more.

Investigations found basic safety norms were flouted - explosives stored in makeshift sheds, weak enforcement, poor compliance and a combination of competitive, community-driven pressure to override regulations, according to a study published in 2018.