SHANGHAI, Sept. 15 -- Chinese officials made their first arrests in connection with an investigation into contaminated milk powder and said two infants have died after drinking it, while 1,253 are known to have fallen ill.

Officials arrested two brothers, surnamed Geng, who are suspected of mixing a substance known as melamine into milk to try to fool buyers who tested its protein content. While the chemical, which is used in the manufacturing of plastics, does not actually add any nutritional value, it boosts readings on tests.

The use of melamine in food products is banned. It is the same substance that had been illegally added to pet food exported to the United States last year that sickened thousands of cats and dogs.

The official New China News Agency said the men sold about three tons of the contaminated milk daily. At least some of the milk is thought to have been sold to Sanlu Group Co., the country's biggest producer of powdered milk.

At a news conference, China's Ministry of Health announced that two babies who had been given the infant formula had died after developing kidney problems. Vice Health Minister Ma Xiaowei said 53 of the infants had severe cases and 340 were hospitalized. The other 913 did not have life-threatening injuries.

The number of cases announced Monday is more than double the 580 victims that were known before.

Officials were first alerted to the problem when an unusual number of babies were being hospitalized for kidney stones.

China's food and drug administration was overhauled last year after a series of embarrassing recalls, but the incident shows that there are still weaknesses in the system.

As the numbers of victims of the contaminated milk powder continue to mount, so does evidence of an attempted coverup.

Chinese officials say complaints about the milk powder go back to March and that Sanlu tests in August discovered that the milk powder contained melamine.

New Zealand dairy cooperative Fonterra, which owns 43 percent of Sanlu, said Sunday that it had asked its Chinese partner to recall the milk powder as early as six weeks ago but Sanlu did not do it until Thursday.

On Monday, New Zealand prime minister Helen Clark said she was told of the problem on Sept. 5 but that local Chinese officials did not order a recall so she informed their bosses in Beijing.

"We were the whistle blowers and they leapt in and ensured there was action on the ground," said Clark, according to the Associated Press. "At a local level . . . I think the first inclination was to try and put a towel over it and deal with it without an official recall."

A Sanlu vice president, Zhang Zhenling, apologized at a news briefing today in Shijiazhuang, capital of Hebei Province, where the corporation is based. "The serious safety accident of the Sanlu formula milk powder for infants has caused severe harm to many sickened babies and their families. We feel really sad about this," he said, according to state media.