KARACHI, Pakistan - A new bridge collapsed in the southern city of Karachi on Saturday, killing at least four people and crushing vehicles under mounds of debris, officials said.

Mayor Mustafa Kamal said officials were using heavy machinery and bulldozers to try to rescue injured people trapped in vehicles when the 100-metre curved section of the overpass crashed onto a road below.

Azhar Faruqi, the city police chief, said authorities had taken two bodies to a hospital, while another two bodies could be seen inside a crushed car. At least five people were injured.

The cause of the accident was not immediately clear.

The bridge had just been inaugurated by President Gen. Pervez Musharraf in August. Prime Minister Shaukat Aziz expressed concern over the quality of the construction and said an investigation would be launched to determine the reasons the structure collapsed.

He pledged to hold responsible those at fault for any shoddy construction.