The Army has assigned two teams of medical investigators to determine the cause of a puzzling spate of pneumonia cases among American troops in Iraq and other countries in the war region, Pentagon officials said yesterday.

From March 1 through July 30, about 100 pneumonia cases have been reported among all troops deployed in Southwest Asia, the officials said, and doctors have not been able to determine the cause of the disease in most of them.

In 15 cases, the pneumonia was severe enough to require attaching the sick troops to mechanical respirators. Ten of the 15 cases occurred in Iraq, where most of the troops are deployed; the others occurred in Kuwait, Qatar and Uzbekistan.

Two have died. Both served in Iraq, and one died there. The other died after being taken to Germany.

Epidemiologists are focusing on environmental factors like dust and dehydration because they have ruled out SARS, Legionnaires' disease, anthrax, smallpox and other infectious agents as a cause of the pneumonias.

Col. Bob DeFraites, the Army's chief of preventive medicine, said at a news conference yesterday that he was "pretty close to ruling out" biological or chemical weapons as a source of the pneumonias.

At the news conference, military officials said the number of cases and deaths was about usual for the size of the population involved. They said they were concerned about the serious cases and deaths despite the small numbers.

About three deaths from pneumonia usually occur among all Army troops each year, said Dr. DeFraites, "so two occurring in one area of the world in about a month was enough to cause us concern" and to lead to an epidemiologic investigation.

Dr. DeFraites said he was constrained from disclosing the total number of troops deployed in the region by military security.

One of the most puzzling aspects of the pneumonia is the sporadic nature of the cases and the lack of a discernable pattern of transmission. "No two of the cases share any common unit or exact day or time," and there is no evidence that it was spread person to person, Dr. DeFraites said.

Fourteen were soldiers, and one was a marine. Two cases occurred in March, two in April, one in May, six in June and four in July, Dr. DeFraites said.

The patients' ages ranged from 19 to 40, with a median of 24, Dr. DeFraites said in an interview. All but one were men, he said. One of the two deaths occurred in June, the other in July. Full autopsy reports are expected in about 10 days.

Dr. DeFraites said his team is trying to obtain data to compare the incidence of pneumonia in the Iraq war to that among troops during the Persian Gulf war in 1991. He said he was also trying to determine if any pneumonia cases have occurred among British troops in Iraq.