WASHINGTON — The Biden administration announced on Thursday that it would restart a program that provided free coronavirus tests to American households through the Postal Service, part of a new push by the government to head off rises in virus cases and hospitalizations as winter approaches.

The program, which was paused in September after distributing over 600 million tests, is restarting after federal officials dipped into what the White House said was “existing, limited funding.” Households will be able to order four tests at covidtests.gov, with shipments beginning next week.

The tests are one component of a broader Covid-19 “winter preparedness plan” the White House rolled out on Thursday, reflecting mounting anxiety over a cold-weather surge in virus cases across the country. Cases have risen roughly 55 percent over the past two weeks, while deaths have surged by around 65 percent in the same period. Hospitalizations have risen over 20 percent, adding strain to medical centers already deluged by cases of the flu and respiratory syncytial virus, or R.S.V.

Two Omicron subvariants, BQ.1 and BQ.1.1, have largely driven the climb in coronavirus cases. They now make up around two-thirds of cases in the United States and can dodge prior immune defenses more easily than other recent variants.

The Biden administration has been urging Americans to update their protection by getting one of the reformulated booster shots introduced in September, but the reaction to the new shots has been tepid. Only about 14 percent of Americans ages 5 and up have received an updated booster. Among those 65 and older, an age group that includes people who are particularly vulnerable to Covid-19, barely a third of people have received one of the new shots.

As part of the winter plan, the administration said it would expand those eligible to vaccinate the elderly to include staff members at nursing homes.

The at-home test program is reopening as lawmakers on Capitol Hill are working toward finalizing a spending package to keep the government funded through September. The Biden administration has asked for more than $9 billion in additional funding for the pandemic response, but Republicans have resisted calls for more Covid-19 spending, arguing that the administration has demonstrated it can repurpose federal funds for virus-related needs.

The new round of at-home tests is being paid for with funds from the $1.9 trillion pandemic relief bill enacted last year, according to a senior administration official who briefed reporters on Wednesday evening on the condition of anonymity. The renewal of the program risks agitating Republicans by suggesting that the federal government still has money it can draw on for the pandemic response, despite its pleas for more.

The Biden administration began offering free at-home tests through the Postal Service in January after demand for coronavirus testing skyrocketed with the arrival of the highly contagious Omicron variant. President Biden was sharply criticized for not moving earlier to secure tests for Americans, even as federal officials argued that they had acted aggressively in procuring them, including for schools and long-term care facilities.

The government continued operating the at-home test program as the year went on, allowing Americans to order additional rounds of tests. But the White House paused the program in September, citing a lack of funding.

Americans with health insurance and on Medicare can still secure eight at-home tests for free each month. As part of its announcement on Thursday, the White House said it would distribute at-home tests to thousands of federally supported senior facilities and at hundreds of food banks.