Sudanese refugees stream into neighboring Chad with stories of rape and killing, while the world struggles to figure out whether yet another genocide is unfolding before it.

Genocide is a word fraught with political meaning, but it also has a strict definition. The 1948 United Nations Genocide Convention defines the crime as the intentional destruction of a religious, racial, national or ethnic group. The treaty requires any government to take action in Sudan if indeed it decides to call the Darfur crisis genocide.

Physician Jennifer Leaning, from the non-profit group Physicians for Human Rights, recently visited Chad's eastern border. She says that while genocide is a legal term, it is up to physicians and humanitarian observers on the front lines of a conflict to get the kind of information that determines whether genocide is actually taking place.

"There is very little guidance in the convention about how you apply its terms," she said. "How do you say in a certain circumstance, 'this has reached a level of severity, intensity, scope, systematic organization of assault to say yes indeed this looks like genocide. Science matters a great deal in this setting.'"

Dr. Leaning says the most scientific way to document the crisis would be to interview random samples of both Sudanese refugees in Chad and those still in Darfur. Some investigators have reached the Chad border. However, she notes that physical danger and political hurdles prevent researchers from getting into Sudan proper. Dr. Leaning says the Sudanese government is not granting visas to investigators, so they must rely instead on reports from those few international aid workers allowed in Darfur.

"Their reports have lots of data on what is happening to people, numbers of people getting killed, and they are spot samples," Ms. Leaning said. "What is necessary is to sum up those samples and then through refugee interviews with people who have escaped, begin to get a sense of the scale of what is happening, the organization of what is happening, and the consequences for the people. And that's where the inquiry comes in."

Physicians for Human Rights is pooling its findings with those of aid workers in Sudan and outside groups. Dr. Leaning says together, they give a broad, if not exact, picture. She estimates that between 30,000 and 120,000 people have already been killed, with roughly one to three million refugees in Chad and Sudan. Dr. Leaning also predicts that about 300,000 more people could die from disease and malnutrition in coming months. She calls it genocide.

"If you're going to wait for absolute certainty then the whole thing is over, and you've lost that group of people, again," she said. "We are seeing enough, based on our partial information, to say that we think that a genocide is unfolding. It's under way."

Not all observers agree. Georgette Gagnon of Human Rights Watch says that the Sudanese government and government-backed Janjaweed militia are committing crimes against humanity, but she does not yet see evidence of intent to destroy a particular ethnic group, a key part of the definition of genocide.

Ms. Gagnon says the definition will be met if scientists in the field find forensic evidence, documents, or specific actions that show intent.

"We have called for an international commission of inquiry to actually start this sort of investigation, by experts," she said. "This is what is needed to determine whether in fact legally, one could put forward allegations of genocide here that would stand up in an international criminal court."

Above all, international human rights groups urge governments and the United Nations Security Council to intervene. In the United States, the Bush administration has not called the Darfur crisis genocide, although some U.S. lawmakers recently introduced resolutions to do so.

"There has been a lot of debate about whether to call what is happening in Darfur genocide," Ms. Gagnon said. "But we know that innocent lives are being lost, we know who's doing it, and we know exactly what will happen if we do not act."