A gas explosion blamed on lax safety measures has killed at least 147 coal miners in China's worst mine accident in three decades, officials said Wednesday.

The Xinhua News Agency, however, did not say exactly what caused the April 21 explosion at the government-owned Sanjiaoje Coal Mine in Shanxi province, west of Beijing. The report also did not say why the accident was not reported until Wednesday.Many Chinese mines lack modern equipment, and accidents are common. In addition to state-owned mines, tens of thousands of tiny, hand-dug mines are operated privately and illegally by peasants.

The Shanxi provincial Coal Industry Bureau said the death toll may be higher because workers were changing shifts at the time of the blast.