Islamabad (CNN) Authorities have been unable to explain the origin of an apparent gas leak that has killed 14 people and left hundreds more sick in southern Pakistan since Sunday.

Concerns are growing in the port city of Karachi in Sindh province that toxic gas could still be leaking as officials scramble to find the source of the problem.

Dr. Zafar Mehdi of Sindh Province's Health Department said 14 people had died from toxic gas exposure in Karachi and that 500 others were affected by the gas to varying degrees, including some who are in intensive care.

Mehdi said autopsies had been carried out on the dead and that post-mortem reports were expected in 72 hours.

Residents in Karachi demonstrate against the government on Tuesday.

Sundus Rasheed, who lives in Karachi's seaside neighborhood of Kemari, told CNN that residents were panicking.

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