Blame games have begun over the failure of the US Senate to pass a new budget and prevent the shutdown of many federal services.
A bill to fund the federal government for the coming weeks did not receive the required 60 votes by the deadline of midnight on Friday.
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U.S President Trump accused Democrats of putting politics above the interests of the American people.
The Democrats in turn blamed him for rejecting bipartisan compromise proposals.
Negotiations in both houses of Congress continued into Saturday with Senate Republican leader Mitch McConnell saying the chamber would be back in session on Sunday to try to end the impasse.
The White House budget chief has expressed optimism that a resolution will be found before Monday.
But if not, hundreds of thousands of federal workers face the prospect of not reporting to work at the start of the week.
The last government shutdown to happen was in 2013, and lasted for 16 days.
Reason for Disagreements
This is the first time a government shutdown has happened while one party, the Republicans, controls both Congress and the White House.
The vote on Friday was 50-49, falling far short of the 60 needed to advance the bill. With a 51-seat majority in the Senate, the Republicans did not have enough seats to pass the bill without some support from the Democrats.
They want funding for border security, including the border wall and immigration reforms, as well as increased military spending.
The Democrats have demanded protection from deportation of more than 700,000 undocumented immigrants who entered the US as children.
The Republicans added a sweetener in the form of a six-year extension to a health insurance programme for children in lower income families. But Democrats want this programme extended permanently.
Mr Trump accused the Democrats of being "far more concerned with illegal immigrants than they are with our great military or safety at our dangerous southern border".
But the leading Senate Democrat, Chuck Schumer, blamed the president, saying Mr Trump was under pressure from "hard-right forces within the administration".
White House spokeswoman Sarah Sanders warned: "The president will not negotiate on immigration reform until Democrats stop playing games and reopen the government."