Media playback is unsupported on your device Media caption Mr Obama accused the Republicans of waging an "ideological crusade"

US President Barack Obama has vowed not to allow Republicans to undermine his signature healthcare legislation as a condition to restart the US government.

The government has partially shut down after the two houses of Congress failed to agree to a new budget, with Republicans insisting on the repeal or delay of Mr Obama's health law.

"They demanded ransom," Mr Obama said.

More than 700,000 federal employees face unpaid leave, and national parks, museums and many buildings are closed.

On Tuesday, Mr Obama blamed conservative Republicans in the House of Representatives for the shutdown, saying "one faction of one party" was responsible because "they didn't like one law".

"They've shut down the government over an ideological crusade to deny affordable health insurance to millions of Americans," Mr Obama said at the White House.

He insisted Congress "pass a budget, end the government shutdown, pay your bills, prevent an economic shutdown".

'The good fight'

Media playback is unsupported on your device Media caption What does shutdown mean for two million federal employees, agencies and tourist destinations?

Republicans, meanwhile, have called for talks with the Democrats.

"Perhaps if President Obama spent less time giving hyper-partisan speeches and more time working with Congress solving problems, we wouldn't find ourselves in this avoidable situation," Rory Cooper, a spokesman for Republican House Majority Leader Eric Cantor, told the BBC.

The White House has rejected a Republican plan to fund only a few portions of the government - national parks, veterans' programmes and the budget of the District of Columbia.

The Obama administration said it would veto any bill to fund the government in part.

"These piecemeal efforts are not serious, and they are no way to run a government," spokeswoman Amy Brundage said in a statement.

Mr Obama did sign a bill on Monday evening to ensure that the military would be paid during the shutdown.

Media playback is unsupported on your device Media caption Furloughed federal worker: "They're all ridiculous"

A spokesman for House Speaker John Boehner called the White House's position "unsustainably hypocritical".

While the Democrats appear united in their opposition to opening discussions on changes to the health law known as Obamacare, signs of fissures have begun to show within the Republican Party.

Representative Scott Rigell broke ranks with Republican leadership and threw his support behind a budget bill that would leave the health law untouched.

"We fought the good fight," he told the New York Times.

And Representative Peter King told the Washington Post he was "the only one who spoke strongly in opposition" to the shutdown, describing his conservative colleagues as "living in their own echo chamber, hearing themselves and talking to each other".

While the Democrats and Republicans blame one another for the morass, a poll released on Tuesday suggested the American public was inclined to fault the Republican strategy.

An estimated 72% of voters oppose Congress shutting down the federal government in order to block the health law, according a poll by Quinnipiac University.

Who is affected?

State department will be able to operate for limited time

Department of defence will continue military operations

Department of education will still distribute $22bn (£13.6bn) to public schools, but staffing is expected to be severely hit

Department of energy - 12,700 staff expected to be sent home, with 1,113 remaining to oversee nuclear arsenal

Department of health and human services expected to send home more than half of staff

The Federal Reserve, dept of homeland security, and justice dept will see little or no disruption

US Postal Services continue as normal

Smithsonian institutions, museums, zoos and many national parks will close US shutdown in 60 seconds Who will be affected Q&A: 2013 US budget brawl

The government ceased operations deemed non-essential at midnight on Tuesday, when the previous budget expired.

National parks and Washington's Smithsonian museums are closed, pension and veterans' benefit cheques will be delayed, and visa and passport applications will go unprocessed.

Couples who had hoped to wed on national park land had to make new plans.

Members of the military will be paid during the shutdown, but base commissaries selling inexpensive and tax-free groceries will close beginning on Wednesday, affecting an estimated 12 million people who shop there.

Programmes deemed essential, such as air traffic control and food inspections, will continue.

Debt ceiling looms

Media playback is unsupported on your device Media caption Engineers with the US Navy talk to the BBC about what they will do during a shutdown: Make skis

Over the past several days, budget bills have bounced back and forth between the House and the Senate, which is controlled by Mr Obama's Democratic Party.

The Democrats have insisted on a "clean" budget bill that would keep the government funded at current levels, while the House Republicans attached a series of measures that would repeal, defund or delay the health law.

Goldman Sachs estimates a three-week shutdown could shave as much as 0.9% from US GDP this quarter.

The healthcare law passed in 2010, was subsequently validated by the US Supreme Court, and was a major issue in the 2012 presidential election that Mr Obama won handily.

One of its major provisions - new online marketplaces for individuals to buy subsidised health insurance - took effect on Tuesday.

As lawmakers grappled with the latest shutdown, the 17 October deadline for extending the government's borrowing limit looms ever larger.

On that date, the US government will reach the limit at which it can borrow money to pay its bills, the so-called debt ceiling.

House Republicans have also demanded a series of policy concessions - including on the president's health law and on financial and environmental regulations - in exchange for raising the debt ceiling.