The US government shutdown prompted growing concern of wider economic consequences as it stretched into a third day.

President Barack Obama challenged Republicans to "stop this farce" by allowing a straight vote on a spending bill.

Both sides in the standoff, which was triggered by Republican efforts to halt Mr Obama's healthcare reforms, appeared entrenched.

Republicans in the House of Representatives are planning at least ten more small bills to reopen specific federal programmes, according to a senior Republican aide.

Democrats reject that piecemeal approach.

Fears grew that the crisis would merge with a more complex fight looming later this month over raising the federal debt limit and that this could stymie any attempts to end the shutdown before the middle of October.

Mr Obama said there were enough Republicans willing to pass a spending bill immediately if House Speaker John Boehner would allow a vote on a spending bill without partisan conditions attached, a so-called clean vote.

But the president said the speaker was refusing to do so because "he doesn't want to anger the extremists in his party."

"My simple message today is 'Call a vote,'" Obama said in a speech at a construction company in Maryland. "Take a vote. Stop this farce, and end this shutdown right now."

Work in Congress was interrupted when the US Capitol was locked down briefly due to gunshots fired outside the building.

In his speech earlier, Mr Obama warned that as painful as theg overnment shutdown was, a default caused by a failure to raise the debt limit would be dramatically worse for the economy as a whole.

Mr Boehner's spokesman said the speaker had "always said that the United States will not default on its debt."

"He also always says that there aren't votes in the House to pass a 'clean' debt limit bill. That's why we need a bill with cuts and reforms," the spokesman said.

Though some moderate Republicans have begun to question their party's strategy, Mr Boehner so far has kept them largely united with the small bills to re-open national parks, restore health research and other parts of the government most visibly affected by the shutdown.

The Tea Party Express, one of the anti-tax groups in the conservative Tea Party that has led the fight against Obamacare, sent an email to supporters yesterday saying that as many as 12 Republicans had indicated they were willing to "give up on the fight" and join Democrats in voting for a funding bill without conditions.

"We need your immediate support to put pressure on the weak Republicans to pass a sensible solution that allows America to avoid the Obamacare train-wreck, while fully funding the federal government," the group said in its email.

The US Treasury warned about the impact of a debt default in a report today, saying a failure to pay the nation's bills could punish American families and businesses with a worse recession than the 2007-2009 downturn.

"A default would be unprecedented and has the potential to be catastrophic: credit markets could freeze, the value of the dollar could plummet, US interest rates could skyrocket," the Treasury said.

"The negative spillovers could reverberate around the world, and there might be a financial crisis and recession that could echo the events of 2008 or worse," it said.

Stock markets worldwide fell while the dollar dropped to an eight-month low over concern the budget standoff would merge with the coming fight over raising the $16.7 trillion US borrowing limit.