BEIJING - A BOMB attack killed four people in central China this week, police said on Saturday, adding that authorities were still investigating the motive behind the blast.

The attack occurred on Monday near a hospital in the city of Zhumadian in Henan province, police said.

'Yes, it happened. Four people were killed, two were injured. It is under investigation,' a Zhumadian police official told AFP by phone, declining further comment.

The Hong Kong-based Information Centre for Human Rights and Democracy in a news release quoted local sources saying an explosive device went off near the entrance of a hospital in the city.

Three people were killed and 40 injured, according to the centre, which regularly sends out reports on instances of social unrest in China that otherwise go uncovered by the country's state-controlled media.

It said the incident may have been carried out by someone unhappy over care at the hospital.

Hospital officials declined detailed comment.

'It's difficulty to say if it's been done by relatives of patients who were not satisfied with our hospital,' an official who declined to give his name told AFP.

China sees thousands of cases of violent social unrest each year, typically as marginalised segments of society lash out over illegal seizures of their land, environmental degradation or government corruption.

In November 2006, large-scale police reinforcements were called in after residents in southwestern China rioted over rumours that a hospital had refused a boy treatment for poisoning because his family could not pay first.

The rumours were later denied by state media, but rights activists said the rioters vented growing discontent over rising medical costs, a lack of health insurance for ordinary people, and hospitals' refusal to treat those who cannot pay.