The US government begins a partial shutdown after the Republican-led House of Representatives refused to approve a budget for next year.
A midnight deadline passed without agreement despite a last-gasp appeal by President Barack Obama.
More than 700,000 US government workers face unpaid leave with no guarantee of back pay once the deadlock is over.
Republicans have insisted on delaying President Barack Obama’s health care reforms as a condition for passing the budget.
It is the US government’s first partial shutdown in 17 years.
With less than one hour to go before midnight, the Republican-led House called for a conference – a bipartisan committee with the Senate to try to thrash out a deal – but Democrats said it was too late to avoid a shutdown.
The White House’s budget office began notifying federal agencies to begin an “orderly shutdown” as midnight approached.
One of the first casualties of the shutdown was the Twitter account for the US Capitol.
“Due to a lapse in government funding, this account will not be active until further notice,” it posted.
Earlier it had warned that the Capitol’s visitors’ centre would be closed if the shutdown went ahead, and all tours would be suspended.
Shortly after midnight, President Barack Obama tweeted: “They actually did it. A group of Republicans in the House just forced a government shutdown over Obamacare instead of passing a real budget.”
House Speaker John Boehner told reporters he hoped the Senate would accept an offer of conference with the House “so we can resolve this for the American people”.
“The House has voted to keep the government open but we also want basic fairness for all Americans under Obamacare,” he said.
The Senate is to meet again at 09:30 on Tuesday, Democratic Senate Majority Leader Harry Reid said.
On Monday afternoon, the Democratic-led Senate voted 54-46 against a bill from House Republicans that would have funded the government only if President Barack Obama’s healthcare law – dubbed Obamacare – was delayed for a year.
The US stock market dropped amid fears of political deadlock, although analysts say serious damage to the economy is unlikely unless the shutdown lasts for more than a few days.
Early on Monday evening, Barack Obama went on national television to criticize Republicans for trying to refight the last election.
A shutdown would have “a very real economic impact on real people, right away,” he said, adding it would “throw a wrench” into the US recovery.
“The idea of putting the American people’s hard-earned progress at risk is the height of irresponsibility, and it doesn’t have to happen.”
After the Senate vote, the chamber’s Democratic majority leader blamed Republicans for the imminent halt to all non-essential government operations.
“It will be a Republican government shutdown, pure and simple,” said Harry Reid, referring to the Republicans as “bullies”.
The House then passed another bill on Monday evening to fund the government – but with a one-year delay to one of the health law’s primary elements not due to begin on October 1, the individual mandate.
The Senate again rejected the Obamacare provisions with less than three hours before the deadline.
Major portions of the healthcare law, which passed in 2010 and has been validated by the US Supreme Court, are due to take effect on Tuesday regardless of whether there is a shutdown.
Under the shutdown, national parks and Washington’s Smithsonian museums will close, pension and veterans’ benefit cheques will be delayed, and visa and passport applications will go unprocessed.
Programmes deemed essential, such as air traffic control and food inspections, will continue.