WASHINGTON — The Obama administration gave conditional approval on Monday to health insurance marketplaces being set up by six states led by Democratic governors eager to carry out President Obama’s health care overhaul.

The six are Colorado, Connecticut, Maryland, Massachusetts, Oregon and Washington.

At the same time, the administration rejected pleas from other states that want to carry out a partial expansion of Medicaid, to cover fewer people than the president and Congress originally intended. Some states want to expand Medicaid to cover childless adults with incomes up to the poverty level, $19,090 for a family of three.

But Cindy Mann, the top federal Medicaid official, said the federal government would pay the full cost of newly eligible Medicaid beneficiaries only if a state raised the threshold to 133 percent of the poverty level — or 138 percent, with an adjustment allowed by federal law. This would guarantee Medicaid coverage for a family of three with income less than or equal to $26,340.

Matt D. Salo, the executive director of the National Association of Medicaid Directors, which represents state officials, summarized the administration’s position this way: “No partial expansion of Medicaid. No phased-in expansion. It’s all or nothing.”