A man has been pulled alive from rubble two days after a factory collapse in Pakistan left 21 people dead.

Sixteen women and four children were among those who perished, officials said, adding that more bodies are still being recovered.

The three-storey building in Lahore was used to make veterinary medical products and is thought to have collapsed after gas cylinders exploded on Monday.

Rescue teams have so far managed to pull out 17 people aliveu00a0-- seven men, six women and four children, he added. A 13-year-old boy was in critical condition after being found some 40 hours after the building collapsed. The latest survivor was discovered 47 hours after the collapse.

"We are told there are still people trapped inside. They are alive and people have heard their cries. We don't know the exact number," said rescue team spokesman Jaber Hussain.

"It's a huge job. We heard the authorities are deploying additional equipment and heavy machinery to clear the debris. The operation may continue late into the night."