Ten women died and 14 were in a serious condition after botched operations at a government mass sterilization "camp" in the central Indian state of Chhattisgarh, officials said on Tuesday.

The women fell ill on Monday, two days after surgery at a so-called family planning camp at a village. Such camps are held regularly in Chhattisgarh and other Indian states as part of a long-running effort to control India's booming population.

"It was a serious matter of negligence. It was unfortunate," the chief minister of the state, Raman Singh, told reporters. Four officials have been suspended and an investigation will be held, he said.

The cause of the deaths was not yet clear, but officials said they were looking into several possibilities, including whether the surgical equipment was infected.

Some 83 women had laparoscopic tubectomy surgery at the camp, receiving incentive payments of 1,400 rupees ($23), said R.K. Bhange, Bilaspur's chief medical officer. Health workers got 200 rupees ($3.25) for each woman they brought to the camp.

The incident will be an embarrassment for Prime Minister Narendra Modi, who has vowed to reform India's health system. Modi, whose Bharatiya Janata Party rules in Chhattisgarh, expressed concern over the tragedy on Tuesday.

Workers of the Congress party, the main opposition party in the state, demanded resignation of the state's health minister and chief minister.

Deaths due to sterilization are not a new problem in India, where more than four million sterilizations were performed in 2013-14, according to the government.

Between 2009 and 2012, the government paid compensation for 568 deaths resulting from sterilization, the health ministry said in an answer to a question in parliament two years ago.

Health advocates worry that paying women to undergo sterilization is dangerous on a number of levels.

"The payment is a form of coercion, especially when you are dealing with marginalized communities," said Kerry McBroom, director of the Reproductive Rights Initiative at the Human Rights Law Network in New Delhi.

She said care at the family planning camps was often inadequate, with surgery rushed through in unhygienic conditions.

Pratap Singh, commissioner of Chhattisgarh's Department of Health and Family Welfare, told Reuters that the state's sterilization program was entirely voluntary.

The United Nations expressed concern.

"If the facts are confirmed, then a grave human tragedy has occurred," said Kate Gilmore, deputy executive director of the U.N. Population Fund. "Where there is deviation from clinical standards, there must be consequences."