By Syed Raza Hassan

ISLAMABAD (Reuters) - At least 12 Pakistani workers were killed and more than 50 injured on Tuesday after a huge blast ripped through an oil tanker at a ship-breaking yard, trapping others inside the vessel, local officials said.

The leader of a ship-breaking workers union said fire had engulfed the vessel moored in Gaddani, 45km (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."

At least 12 people were killed and another 58 injured in the blast, said Zulfiqar Bokhari, a senior administrative official in the area.

Nasir Mansoor, a representative of the National Trade Union Federation of Pakistan, said the explosion sent pieces of metal flying up to two kilometers (one mile) away.

The blast occurred in the fuel tank of the ship, which was still on fire, he said.

Firefighters from Karachi and from the air force and navy were attempting to put out the blaze, he said.

Mansoor said the firefighters would have to wait for the fire to die out "as they lack the foam required douse it."

Laborers 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 its boom years at the end of the last decade.

(Additional reporting by Gul Yusufzai in QUETTA; Editing by Drazen Jorgic; Editing by Robert Birsel)