The federal government has told New York State health officials that chemotherapy, which had been covered for illegal immigrants under a government-financed program for emergency medical care, does not qualify for coverage. The decision sets the stage for a battle between the state and federal governments over how medical emergencies are defined.

The change comes amid a fierce national debate on providing medical care to immigrants, with New York State officials and critics saying this latest move is one more indication of the Bush administration’s efforts to exclude the uninsured from public health services.

State officials in New York and other states have found themselves caught in the middle. The New York dispute, focusing on illegal immigrants with cancer — a marginal group of unknown size among the more than 500,000 people living in New York illegally — has become a flash point for health officials and advocates for immigrants in recent weeks.

Under a limited provision of Medicaid, the national health program for the poor, the federal government permits emergency coverage for illegal immigrants and other noncitizens. But the Bush administration has been more closely scrutinizing and increasingly denying state claims for federal payment for some emergency services, Medicaid experts said.