ISLAMABAD: At least 10 Pakistani workers were killed and more than 50 injured on Tuesday by huge blast in an oil tanker at a ship-breaking yard, trade union officials said, adding that the death toll could rise.

The leader of a ship-breaking workers union said a big explosion and fire engulfed the vessel moored in Gaddani, 45 km (28 miles) northwest of the port city of Karachi.

"Things are really bad here," union president Basheer Mehmoodani told Reuters by telephone.

"There's an unclear number of workers said to be trapped in the burning ship."

It was not clear what caused the blast.

Firefighters were called from Karachi but they proved ineffective given the size of the blaze, he said.

Nasir Mansoor, a representative of the National Trade Union Federation of Pakistan, said the explosion was so big it blasted pieces of metal two kilometres (one mile) away.

"The blaze is still raging and there is nothing fire-fighters can do except to wait for it to die out as they lack foam required douse it," Mansoor said.

Labourers in Gaddani often work in poor conditions without basic protective gear.

The Gaddani ship-breaking industry has fallen on hard times recently and employs about 9,000 workers, fewer than in boom years at the end of the last decade.