WASHINGTON (AFP) - US President Barack Obama on Tuesday slammed Republicans for shutting down the government as part of an "ideological crusade" designed to kill his signature health care law.

"I urge House Republicans to reopen the government," Obama said at the White House, as he highlighted a major portion of the health reform program which came into force on Tuesday.

The first government shutdown in 17 years took place because House of Representatives Republicans repeatedly made funding for government operations for the new fiscal year contingent on defunding, delaying or dismantling Obamacare.

Each attempt was turned back by the Democratic-led Senate, which must also agree budget legislation.

But the clock on already agreed government financing ran out at midnight.

Hundreds of thousands of federal workers faced layoffs, and national parks and large swathes of government operations were closed down.

"This Republican shutdown did not have to happen - I want every American to understand why it did happen," Obama said in the White House Rose Garden.

"They have shut down the government over an ideological crusade to deny affordable healthcare to millions of Americans."