A car bomb exploded outside a mosque on Sunday, killing 37 people and wounding another 141 in a Shiite Muslim dominated neighbourhood in the southern Pakistan city of Karachi, the third mass casualty attack on the minority sect in the country this year.

No one has taken responsibility for the bombing, but Shiite Muslims have been increasingly targeted by Sunni militant groups in Karachi, Pakistan's economic hub and site of years of political, sectarian and ethnic violence, as well as other parts of the country.

The bomb exploded outside a Shiite mosque as people were leaving evening prayers in Pakistan's largest city. Initial reports suggested the bomb was rigged to a motorcycle, but a top police official, Shabbir Sheikh, said later that an estimated 100 kilograms of explosives was planted in a car.

Col. Pervez Ahmad, an official with a Pakistani paramilitary force called the Rangers, said a chemical used in the blast caught fire and spread the destruction beyond the blast site. Several buildings nearby were engulfed in flames.

Men and women wailed and ambulances rushed to the scene where residents tried to find victims buried in the rubble of collapsed buildings. The blast left a crater that was 2 metres (yards) wide and more than 1 metre (4 feet) deep.

"I was at home when I heard a huge blast. When I came out, I saw there was dust all around in the streets. Then I saw flames," said Syed Irfat Ali, a resident who described how people were crying and trying to run to safety.

A top government official, Taha Farooqi, said at least 37 people were confirmed dead and 141 more were wounded.