The US Congress has missed the deadline for averting the first partial government shutdown in 17 years.
As the clock struck midnight, House Republicans were demanding that the Senate negotiate their demand for a one-year delay in making millions of people buy health insurance under President Barack Obama's 2010 health care law.
Minutes before midnight, the White House ordered a shutdown.
The Democratic Senate on Monday twice rejected GOP demands to delay key portions of what has become known as Obamacare as a condition for keeping the government open.
An estimated 800,000 federal workers faced enforced absences, although many were told to work a half-day on Tuesday.
Critical functions such as air traffic control and military operations will continue. Social Security benefits will be paid. National parks and most federal offices will close.
Mr Obama told members of the military he would work to get Congress to re-open the government as soon as possible.
The president addressed troops in a video message after Congress missed the midnight deadline.
Mr Obama said troops in uniform would remain on duty as usual. He said he had signed a law ensuring troops get paid on time.
He said ongoing operations in Afghanistan and elsewhere would continue and the US would ensure those in harm's way would have what they need.
But he said Defence Department civilians may face enforced absences.