Credit…Leigh Vogel for The New York Times

The White House said on Tuesday that with no new coronavirus funding on the horizon, it was already scaling back its plans to purchase monoclonal antibody drugs to prevent and treat Covid-19 and will stop reimbursing medical providers providing Covid care to the uninsured in early April unless more money is approved.

As top administration officials appealed for $22.5 billion in additional federal funding, the prospects for an emergency aid package looked bleak on Capitol Hill. Republican No. 2 in the Senate, John Thune of South Dakota, said a bill to provide the money would be “much heavier” if the administration did not come up with ways to offset the cost.

Republican senators, demanding better accountability for how the Biden administration has already spent hundreds of billions on pandemic relief, are digging in. They want it paid for by reallocating funds from other programs.

Sen. Roy Blunt of Missouri, the top Republican on the committee that oversees health care spending, said he spoke to Jeffrey D. Zients, President Biden’s coronavirus response coordinator, on Tuesday about the standoff. Like many other Republican senators, he said he supports the purchase of more therapeutics, tests and vaccines – but with a catch.

“My advice to them is to be as transparent as possible with what money is left and where it would be spent, whether it’s being spent as designated, and how much money you need, and how much long it will last,” said Mr. Blunt.

Senior administration officials, speaking on a conference call with reporters, reiterated their original demand for the full $22.5 billion in new money. The amount was cut by Congress to $15.6 billion, but even that is now in play. Lawmakers removed Covid relief funding from the spending bill they passed last week due of a dispute over whether $7 billion of the money should come from state funds.

The same day the administration renewed its push for more funds, a Senate panel voted to create an independent commission to investigate the causes of the pandemic and the federal response. It’s part of a larger package of pandemic measures that the panel voted to send to the full chamber.

With coronavirus cases rising in parts of Europe, some Asian countries experiencing severe outbreaks and public health experts warning of the possibility of another variant or a summer or fall surge in the US , the Biden administration is increasingly concerned that without more money, they will be caught off guard.

President Biden’s new coronavirus response plan, outlined earlier this month, included plans to get more antiviral pills; strengthen testing capacity; and to accelerate the development of the next generation of vaccines, in hopes of being able to protect against multiple variants. Without congressional funding, officials warned, those plans are in jeopardy.

White House officials have repeatedly said they have run out of money for vaccines, tests and treatments. During Tuesday’s call, administration officials, who declined to be identified by name, provided more details than in the past. They said the administration wanted to place new orders for monoclonal antibody treatments — including Evusheld, a drug licensed to protect Americans at high risk from Covid-19 — by the end of March, but would not be able. to do so without additional funding.

The federal government buys the treatments from the manufacturers and provides them free to the public. But in order to keep treatments free for as long as possible, senior officials said, the administration plans to reduce quantities shipped to states starting next week.

On Capitol Hill, the fate of the Covid relief bill is unclear. House Democrats considered putting the measure to a vote this week, but it was unclear whether they would if the measure was not guaranteed to pass the Senate.

House Democrats initially thought they had answered Republicans’ objections by taking money from other programs to offset the $15.6 billion, including $7 billion from state relief funds. But governors balked, prompting House Speaker Nancy Pelosi to withdraw Covid funding from the comprehensive $1.5 trillion spending package that was approved last week.