CHONGQING -- The death toll has risen to seven after a bus was smashed by a huge rock that came barreling down the side of a mountain on a country road in southwest China's Chongqing Municipality on Thursday.

The bus, carrying 22 people, was heading from Xintian Township to Cizhu Township in Wanzhou District, said a spokesman of the Cizhu Township government.

Six people were killed on the spot and another one died after an emergency operation failed.

The injured are being treated at a hospital in Wanzhou District.

The rock, measuring about one cubic meter, raced down the mountain and smashed into the rear of the bus.

"The bus was not overloaded," said the spokesman.

He attributed the accident to continuous rain that may have loosened soil and rocks on the mountainside.