WASHINGTON (AP) — A government report says the cost of expanding Medicaid to millions more low-income people is increasing faster than expected.

That’s raising questions about a vital part of President Barack Obama’s health care law.

The law called for the federal government to pay the entire cost of the Medicaid expansion from 2014 through the end of this year.

Obama has proposed an extra incentive for states that have not yet expanded Medicaid: three years of full federal financing no matter when they start. But the new cost estimate could complicate things.

In a recent report to Congress, the Centers for Medicare and Medicaid Services said the cost of expansion was $6,366 per person for 2015, about 49 percent higher than previously estimated.

The Obama administration says those estimates are preliminary.