An explosion in a factory in China's eastern Zhejiang province has caused casualties, the official Xinhua news agency reports.

The blast happened at around 9am on Sunday in Ningbo, a major port and industrial hub, south of Shanghai on Hangzhou Bay.

The blast caused some buildings to collapse, Xinhua said, quoting the city's fire department.
It was unclear what caused the explosion.

Several buildings collapsed in a nearby residential area that had already been cleared of people, according to Zhejiang Daily. Police told Reuters the area had been marked for demolition.

Rescue work and investigation into the cause were underway, police said in a statement issued on its verified Weibo account.

Pictures circulating on both state and local media showed a street littered with debris and shattered glass and the arrival of several fire engines. Reuters could not independently verify the authenticity of the photographs.

Blasts and other accidents are common in China due to patchy enforcement of safety rules, although the government has pledged to improve checks to stamp out such incidents.