U.S. President Barack Obama is blaming Republicans on an "ideological crusade" to kill his signature health care law for shutting down the federal government.
Much of the U.S. government is closed for business after Congress failed to agree on a temporary spending bill to keep it operating.
The Republican majority House of Representatives repeatedly linked money for government operations for the new fiscal year to defunding, delaying or dismantling the health insurance law known as "Obamacare." Each attempt was turned back by the Democratic-controlled Senate, which must also agree to budget legislation.
The Senate also rejected a House request for negotiations on the matter.
About 800,000 federal workers are on furlough, while other federal workers are staying on the job with no guarantee when they will be paid.
The shutdown will not affect Voice of America broadcasts, but will close national parks, traffic safety agencies, and lay off most Defense Department civilian employees. The U.S. military will remain on duty, and ongoing military operations like those in Afghanistan will continue.
Homeland Security agents and border security offices will remain open, as well as other law enforcement agencies. Enrollment for "Obamacare" also began as scheduled Tuesday.
House of Representatives Speaker John Boehner said he was disappointed the Senate rejected a House bill to fund the government through December 15. He said the bill would "provide fairness to the American people under Obamacare - no exemptions, no exceptions, let's treat everyone the same." It also included Boehner's call "to sit down, discuss and try to resolve those matters.''
Speaking in the White House Rose Garden, Mr. Obama said the longer the shutdown continues the worse the impact will be.
Experts say a shutdown of more than two weeks is likely to slow the U.S. economy through reduced tourism cash flow and furloughed federal workers curtailing their personal spending.
If Congress can agree to a new funding bill soon, which Mr. Obama insists not be tied to health care reforms, the shutdown would last days rather than weeks, with relatively little impact on the world's largest economy. But no signs of compromise have emerged from the Senate.