WASHINGTON — Doctors are improperly billing poor people on Medicare for deductibles, copayments and other costs from which they are supposed to be exempt, the Obama administration says.

Federal officials have warned doctors that they may be subject to penalties if they persist in these practices. They could, for example, be fined or excluded from Medicare.

The people who are being billed improperly are "qualified Medicare beneficiaries" who are also enrolled in Medicaid. They are 65 and older or disabled and have low incomes, generally less than $1,010 a month for an individual or $1,355 for a married couple.