(Adds comments from police)

KARACHI, Jan 9 (Reuters) - Fire swept through a slum in the Pakistani city of Karachi early on Friday killing 39 people, including 20 children, officials and witnesses said.

The fire broke out at about 1 a.m. (2000 GMT) and quickly engulfed about 40 huts erected on a vacant piece of land surrounded by buildings in a congested city neighbourhood.

"We were fast asleep when the fire broke. It spread quickly through the entire area," said resident Maqsood Ahmed, who lost three members of him family in the blaze.

Provincial Health Minister Sageer Ahmed said the blaze was believed to have been started by a fire some residents had lit outside a hut to keep warm on a chilly night.

A local police official, Ameen-ur-Rehman, told Reuters many people died because the shanty was hemmed in on three sides by the surrounding walled buildings.

"They only had the western side to run, and that caused more deaths," Rehman said.

(Reporting by Imtiaz Shah and Aftab Borka; Writing by Augustine Anthony; Editing by Robert Birsel and Alex Richardson)

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