ISLAMABAD, June 3 (Xinhua) -- At least four laborers were killed and nine others injured in a coal mine explosion in Quetta of Pakistan's southwest Balochistan province on Sunday, local reports said.

According to initial reports, the explosion caused by the accumulation of poisonous gas in the mine, Dawn News said.

The mine partially caved in following the explosion, trapping the workers inside the debris. Rescue teams later recovered the four bodies and the nine injured people from the rubbles.

The injured miners were shifted to a nearby hospital where three of them are said to be in critical condition.

Unsafe working conditions are blamed for the frequent mine accidents in Pakistan. Earlier last month, 27 laborers were killed and scores others injured in two incidents of coal mine explosion in Balochistan.