A bomb blast that killed eight people and injured dozens outside a kindergarten in eastern China was the work of a 22-year-old introvert who was among the dead, authorities said Friday.

The public security ministry said in a statement that the suspect was a man with health problems who rented a room near the school in Fengxian, where Thursday's blast occurred.

Police found material to make a homemade explosive device at the apartment, which had the words "die", "death" and "destroy" written on the wall.

The statement said the man, surnamed Xu, suffered from dysfunction of the autonomic nervous system, a disorder that can cause heart and blood pressure problems.

Liu Wenxiang, a police investigator, said on CCTV state television that Xu worked at a restaurant in Fengxian after he was suspended from school in May 2016.

Liu described Xu as "introverted and withdrawn" when interacting with others.

The motive was not immediately known and police did not say whether the kindergarten was the target, but schools have been attacked by knife-wielding assailants in the past.

The explosion in Jiangsu province left victims bleeding and weeping, with images posted on state media showing some had their clothes torn off by the force of the explosion.

A survivor told broadcaster CCTV from her hospital bed that the school gate had just opened for kids to leave, though none had come out when the fireball erupted.

The woman, who was holding a baby during the interview as she sat on her bed, said she blacked out and only woke up while being treated.

The Fengxian county government said class had not yet been dismissed and that no teachers or students were among the casualties.

At least eight people were killed and 65 injured, including eight seriously, according to authorities.

AAP

'So scary'

Pictures of the scene showed more than a dozen people outside a building, many lying on the ground and some appearing to be unconscious, including a small child.

Locals said hundreds of students attend the school and that the victims were thought to be people waiting to pick up the children.

"It's so scary," said a man surnamed Meng, who learned about the explosion online.

0:00 Scenes from the China kindergarten blast. 00:00 / 00:00 Share Share on Twitter

Share on Facebook

Investigators concluded that the blast was caused by an explosive, ruling out previous news reports suggesting it was an accidental explosion of a food stall's gas cylinder.

On Friday, police officers wearing blue plastic around their shoes scoured the scene of the explosion in front of the kindergarten.

The education ministry later urged schools to strengthen security risk prevention.

Eight killed, dozens hurt in China kindergarten blast Major parties throw stones on China links

History of attacks

There have been several cases in China of disgruntled individuals setting off explosions or starting fires in revenge for perceived wrongs.

In May, a school bus packed with kindergarten pupils erupted in flames inside a tunnel in eastern Shandong province on May 9, killing 11 children, a teacher and the driver.

Officials later said the fire was intentionally set by the driver, who was angry at losing overtime wages.

A 29-year-old man with gambling debts tossed explosives in Shanghai's main airport last year, injuring four people.

In 2015, an unemployed man set off an explosion at a public park in the eastern province of Shandong, killing himself and another person and wounding 24.

In 2013, a wheelchair-bound man detonated a homemade bomb at Beijing international airport as a protest at alleged police brutality.

There have also been knife attacks at schools in recent years, the most recent being in January, when a man armed with a kitchen knife wounded 11 children at a kindergarten in southern China.