The US government is bracing for a possible shutdown, as Republicans and Democrats in Congress remain deadlocked on a budget to continue its funding.
Agencies have begun making contingency plans ahead of the 1 October deadline to pass a new funding resolution.
The Senate has passed a bill to fund the government until 15 November.
But House Republicans have said they refuse to approve the bill without a provision to strip funding from President Barack Obama's health law.
The Senate is controlled by Mr Obama's Democratic party, while the Republicans hold the majority in the House of Representatives.
As a result, lawmakers are at a stalemate as the deadline approaches.
Government agencies have been selecting workers considered essential should funds stop flowing.
Obama exhorts conservatives
The looming shutdown is one of two fiscal crises facing the US government. On 17 October, the US treasury department's authority to borrow money to fund its debt obligations expires unless Congress approves a rise in the so-called debt ceiling.
On Friday afternoon, President Barack Obama urged House Republicans to pass the Senate's stopgap budget bill and to extend the debt limit, and demanded they not threaten to "burn the house down because you haven't gotten 100% of your way".
Mr Obama said if the nation were to default on its debt, it would have a "profound destabilising effect" on the world economy.
"Voting for the treasury to pay its bills is not a concession to me," he said. "No-one gets to hurt our economy... just because there are a couple of laws [they] don't like."
He described the healthcare law as "a done deal" and said the Republican-backed repeal effort was "not going to happen".
Mr Obama said the Senate had "acted responsibly" in passing the budget measure and that now it was up to Republicans in the House of Representatives "to do the same".
If the government does shut down on 1 October, as many as a third of its 2.1 million employees are expected to stop work - with no guarantee of back pay once the deadlock is resolved.
National parks and the Smithsonian museums in the nation's capital would close, pension and veterans' benefit cheques would be delayed, and visa and passport applications would be stymied.
Programmes deemed essential, such as air traffic control and food inspections, would continue.
The defence department has advised employees that uniformed members of the military will continue on "normal duty status", but "large numbers" of civilian workers will be told to stay home.
Last week, the US House of Representatives approved a bill that would maintain the US government's funding levels until 15 November but strip funding from Mr Obama's health law, known as Obamacare.
On Friday the Senate passed a version of the bill with the defunding provision removed 54-44, largely on party lines.
"The Senate has acted and we've done it with bipartisan co-operation. We've passed the only bill that can avert a government shutdown Monday night," Democratic Senate leader Harry Reid said.
"This is it, time is gone."
The House is now expected to take up that bill at the weekend. Unless the two chambers can come to a consensus and pass a bill for Mr Obama to sign, the federal government will close on 1 October.
Analysts say House Speaker John Boehner and his leadership team are pushing for the chamber to approve the Senate-passed bill and move on to the debt limit fight next week.
But more conservative members of his restive Republican caucus object, hoping to use the threat of a shutdown as leverage to force a halt to Mr Obama's health law.
That law passed in 2010, was subsequently ruled constitutional by the US Supreme Court, and was a central issue in the 2012 presidential election won by Mr Obama.
After the Senate vote on Friday, Republican Senator Ted Cruz and two other conservative senators denounced the result and vowed to press on with their effort to get rid of Obamacare.
"This will not end here," Senator Marco Rubio told reporters.
Meanwhile, wrangling over the debt limit extension continues, with the Republicans seeking to win a series of major policy concessions by tying them to an increase.
On Thursday, the number-two Republican in the House, Majority Leader Eric Cantor, said the party would introduce a bill extending the debt ceiling for a year - but also delaying for a year major provisions of Mr Obama's health law.
While Congress narrowly avoided a government shutdown in 2011, the high-stakes political wrangling has become especially chaotic.
Analysts point to infighting in the Republican Party caucuses in both the House and Senate.
Republican Senator John McCain told CBS News on Friday he had never seen such dysfunction in Congress in his three decades as a senator.
"We are dividing the Republican Party rather than attacking Democrats," he said.