The US government shut down at midnight on Friday after Democrats and Republicans failed to reach a last-minute deal to fund its operations, divided in a bitter dispute over immigration and border security.
In a dramatic late-night session, senators blocked a bill to extend government funding through to February 16. The bill needed 60 votes in the 100-member Senate but fell short, with only 50 supporting it.
Most Democrats opposed the bill because their efforts to include protections for hundreds of thousands of mostly young immigrants known as Dreamers failed.
Huddled negotiations by Senate Majority Leader Mitch McConnell and Senate Democratic leader Chuck Schumer in the last minutes before midnight were unsuccessful, and the US government technically ran out of money at midnight.
The shutdown formally began on Saturday, the first anniversary of President Donald Trump's inauguration.
Trump immediately sought to blame Democrats.
"Tonight, they put politics above our national security, military families, vulnerable children, and our country’s ability to serve all Americans," the White House said in a statement.
It also said it would not discuss immigration until the government is up and running again.
"We will not negotiate the status of unlawful immigrants while Democrats hold our lawful citizens hostage over their reckless demands. This is the behavior of obstructionist losers, not legislators."
In return, Schumer pointed the finger directly at Trump.
"It's almost as if you were rooting for a shutdown and now we'll have one and the blame should crash entirely on President Trump's shoulders," he said.
Until a funding deal is worked out, scores of federal agencies across the country will be unable to operate, and hundreds of thousands of "non-essential" federal workers will be put on temporary unpaid leave.