Three Pakistani Christian sewage workers have died of suffocation while cleaning a blocked sewage pipeline in southern Punjab.

Saleem Masih, 50, Danish Masih, 20, and Nadeem Masih, 18, are believed to have been untrained and without any protective apparatus.

On July 3 Saleem and Nadeem died shortly after jumping into the pipeline while Danish was pulled out alive by people standing around the manhole but died on the way to hospital.

Sewage pipelines generate a number of harmful gases, including methane, ammonia, hydrogen sulphide, carbon monoxide, sulphur dioxide and nitrogen oxides.

The sewage gases can be deadly to those who work on ewers or treatment plants.

None of three men had been wearing any king of protective clothing or apparatus, something that is common among Pakistani sewage workers.

They work unsupervised and are not provided with any training.