The collapse of talks between President Donald Trump and Republican leaders to keep the government funded temporarily left dozens of federal agencies facing the prospect of closing at midnight Friday, with officials offering scant details on what operations would continue on the eve of the Christmas holiday weekend.

A partial shutdown seemed increasingly likely, with hopes dwindling of an agreement to fund the Departments of State, Agriculture, Interior, Treasury, Justice, Commerce, Homeland Security, Transportation and Housing and Urban Development and the Environmental Protection Agency, as well as dozens of smaller independent agencies.

Trump says he refuses to sign a short-term spending bill passed by the Senate on Wednesday unless it includes money for a wall along the Mexican border; that vow left agencies scrambling to set in motion their plans to cease - or partially cease - operations. But many federal managers were preparing to leave for the holiday, and the White House was not making public any details of what would stay open and what would close.

While the Office of Management and Budget held a conference call with agency leaders last week to notify them to begin shutdown preparations, it is leaving it up to each agency to execute its plans.

Agencies were encouraged to use extra money, if they have it, to try to stay open for a short period. But late Thursday, officials had not spelled out which offices would be able to keep operating and for how long.

Many national parks would be accessible to the public, for example, as they were during a brief shutdown last January, but their visitor centers and restrooms would be closed and there would be no trash collection or park rangers on duty. Historic homes and areas of parks where roads cannot be plowed would be closed. Roads and trails and open-air memorials would stay open.

Most employees who are likely to be sent home without pay were not expected to be notified until Friday, when many of them will be on vacation, leaving managers rushing to come up with plans to notify them.

"At the operational level, our people have not been given instructions for how to wind down," said Jacque Simon, policy director for the American Federation of Government Employees, the largest union representing federal workers.

"Right now, everyone is just acting in a state of uncertainty," Simon said. "They were operating on the assumption that Trump was going to sign something."

With the government already closed for Christmas on Monday and Tuesday, and so many staffers planning to be away the rest of the week, bosses were preparing for how to contact employees in case they needed to officially inform them of staffing plans.

The reality was that it could be Wednesday before many agencies would be able to start official preparations for a shutdown, during which furloughed workers must come to work briefly to turn in their government-issued laptops and cellphones and change their voicemail greetings.

Employees who have been approved for days off but who are deemed essential would be called back to work.

About 75 percent of the portion of the federal budget controlled by Congress has been funded through September, including the Pentagon, Veterans Affairs and the Health and Human Services Department. That leaves about a quarter of the government - and more than one-third of federal workers - that would have no money appropriated to operate.

The unfunded agencies include the Peace Corps, the Small Business Administration, the General Services Administration, the National Archives and NASA.

About 800,000 of the 2.1 million federal employees across the country would be affected. About 47 percent of them would be sent home without pay, with the rest staying on the job because their work is considered essential to the functioning of the government, according to contingency plans the agencies have filed with the Office of Management and Budget.

Employees who are called to work during a shutdown do get paid, but not until the government reopens. Those who are furloughed are not guaranteed to be paid once the government reopens. But after every previous shutdown, Congress has passed legislation mandating that they be paid.

A shutdown also would affect thousands of government contractors whose employees work side by side in the same offices, airports, hospitals and research labs as federal workers. While some contractors would continue to work, many others would be sent home and could not work remotely. And some of them would not be reimbursed by their companies, if past shutdowns are any guide.

The EPA's acting administrator, Andrew Wheeler, notified the staff late Thursday that he had found enough reserve funds to keep the agency open through Dec. 28.

Still, confusion reigned early in the day as supervisors began gathering personal email addresses, phone numbers and home addresses for their staffs if they had to reach them during the holidays, two employees who were not authorized to speak publicly said.

The Interior Department, meanwhile, planned to use the department's email system to notify employees who would be furloughed and who will stay working - for example, watching over sensitive operations such as wildlife hatcheries and keeping clean water flowing to counties.

But that strategy seemed risky since federal employees are not allowed to check their email when the government is closed.

The State Department says it plans to maintain passport and visa services, which generate fees that will allow the offices to remain open as long as there are sufficient funds. The main exception would be if a passport office were located at another agency that is closed by the shutdown. U.S. embassies and consulates overseas also will remain open to serve American citizens.

During the last government shutdown, some nonessential personnel were told not to come to work, a pattern that is expected to be replicated Monday in the event of a shutdown. But much of the work of diplomacy will continue with enough employees on the job to respond to crises worldwide.

NASA spokesman Steve Coll said the agency is still evaluating what would happen. In the past, NASA maintained personnel to support the International Space Station and its crew, as well as currently operating space missions - such as the InSight lander, which arrived at Mars last month.

But the majority of the agency's 17,500 employees would not be allowed to come to work. For scientists who study supernovas, black hole collisions and other cosmic events that proceed without regard to the status of the federal budget, this raised the fearsome prospect of missing out on interesting science.

Alice Harding, an astrophysicist at Goddard Space Flight Center who studies the swiftly spinning cores of collapsed stars, said she and her colleagues have just spotted a particularly interesting object they want to observe with NASA's Neutron Star Interior Composition Explorer. They've been scrambling this week to submit an observation request so the instrument can track the fleeting phenomenon even if its human operators must stay home.

"We don't know what's going to happen on that," she said.

The National Gallery in Washington planned to remain open to the public using reserve funds until at least Dec. 31, the museum said.

But at Alcatraz Island, managed by the National Park Service, Antonette Sespene, a spokeswoman for the company that runs ferries there, said they might not be able to notify customers of canceled operations until Saturday.

"It'll be very disappointing if we do in fact have to shut down, but our crew is prepared and we'll do our best in communicating (the Park Service's) instructions as quickly as we can," she wrote in an email.

Darryl Fears, Carol Morello, Juliet Eilperin and Sarah Kaplan contributed to this report.