At least seven people were killed and 59 injured in an explosion at the front gate of a kindergarten in eastern China as relatives were picking up their children at the end of the school day, a city government official said.

The Xuzhou city government in Jiangsu province said the blast at the Chuangxin Kindergarten in Fengxian, which struck at 4:50pm Thursday, was under investigation.

It wasn't immediately clear whether the explosion was deliberately set or the result of an accident.

Videos purportedly from the scene and posted on social media showed children and adults lying on the ground, some bleeding. Clothing, shoes and other items were strewn on the ground beside pools of blood.

A cellphone camera video posted on the website of the official People's Daily newspaper showed more than a dozen people lying motionless in front of the kindergarten's steel sliding gate, with clothing, shoes and other items strewn on the ground beside pools of blood.

It showed ambulances arriving and medics wheeling people into an emergency room.

It wasn't clear whether the blast was an accident or had been deliberately set. The newspaper Xiandaikuaibao on its website cited an unidentified witness as saying the explosive appeared to have been a bottle of cooking gas.

Calls to the kindergarten and local hospitals rang unanswered.

Kindergartens in China have been targeted before in apparent revenge attacks carried out by people bearing grudges against their neighbours and society.

This has prompted a tightening of security around many schools, with the posting of guards equipped with truncheons and the installation of gates and other barriers.

China maintains tight control over firearms and most attacks are carried out using knives, axes or homemade explosives.