An explosion at a fireworks plant in a central Chinese province killed at least 21 people and injured 61 others, state media reported Tuesday.

China’s official news agency Xinhua said the blast occurred around 4:40 p.m. local time Monday at a fireworks plant in Changsha city in Hunan province.

State media China Daily said the plant was operated by the Huasheng Fireworks Manufacturing and Display Co. in the Changsha-administered, county-level city of Liuyang, a prominent fireworks powerhouse.

Aerial footage from state broadcaster CCTV on Tuesday showed white smoke still billowing in certain areas of the site, with facilities collapsed or damaged.

Nearly 500 rescuers were deployed to the scene and people within a three-kilometre danger zone were evacuated by authorities, citing high risks from two gunpowder warehouses at the site, the report said.

Authorities were investigating the cause of the blast, and rescuers were spraying down the area to try to prevent further explosions and fire.

They also deployed three robots to help with the search and rescue operation.

Chinese President Xi Jinping urged “all-out efforts” to search for people who were still unaccounted for. He called on authorities to investigate the cause swiftly and pursue serious accountability, the state media report said.

In February, China reported two deadly explosions at fireworks shops around the Lunar New Year period.

Last year, China exported $1.14 billion US worth of fireworks, more ​than two-thirds of global sales, data from the Observatory of Economic Complexity shows.