BILASPUR, India (REUTERS) - Tablets linked to the deaths of more than a dozen women who visited a sterilisation camp in eastern India are likely to have contained a chemical compound commonly used in rat poison, a senior official in Chhattisgarh state said on Saturday.

Preliminary tests of the antibiotic ciprocin tablets were found to contain zinc phosphide, Siddhartha Pardeshi, the chief administrator for the Bilaspur district told Reuters.

The antibiotics were handed out at the mass sterilisation held a week ago in the impoverished state. At least 15 women have died, most of whom had attended the camp.

Mr Pardeshi said authorities had tested the tablets after being informed that zinc phosphide was found at the nearby factory of Mahawar Pharmaceuticals, a firm at the centre of investigations into the deaths at a government-run family planning camp.

Mr Pardeshi said samples had been sent to laboratories in Delhi and Kolkata to verify that the tablets were contaminated as the preliminary report suggested. "But, this is what we anticipate," he said. "Symptoms shown by the patients also conform with zinc phosphide (poisoning)."

An investigation is now under way into why the drugs were bought locally when there was enough stock of the medicine with the state's central procurement agency, Chhattisgarh Health Minister Amar Agarwal said. "There was no incentive to procure locally so we need to investigate why it was done. This means something is wrong," he said.

More possible victims arrived at hospitals from villages on Thursday and Friday, some clutching medicine strips from Mahawar and complaining of vomiting, dizziness and swelling, a doctor at the district's main public hospital said on Friday.

The new patients had not attended the sterilisation camps, but had consumed the drugs separately, the doctor and another official said.

The state government said it had seized 200,000 tablets of Ciprocin 500 and over 4 million other tablets manufactured by Mahawar.

Police have arrested Mr Ramesh Mahawar, the firm's managing director, and his son. Mr Mahawar has said both are innocent.

India is the world's top steriliser of women, and efforts to rein in population growth have been described as the most draconian after China. Indian birth rates fell in recent decades, but population growth remains among the world's fastest.

Sterilisation is popular because it is cheap and effective, and sidesteps cultural resistance to and problems with distribution of other types of contraception in rural areas.