WASHINGTON (AFP) - US Treasury Secretary Jack Lew warned on Sunday that Congress was "playing with fire" as Republicans raised the threat of a US default if President Barack Obama refuses to negotiate on spending.

Speaker John Boehner said the Republican-controlled House would not raise the US debt ceiling without concessions from the White House, staking his position as the government shutdown dragged into day six.

"We're not going to pass a clean debt limit increase," Mr Boehner said in an interview on ABC's This Week. "I told the president, there's no way we're going to pass one. The votes are not in the House to pass a clean debt limit. And the president is risking default by not having a conversation with us," he said.

Mr Lew said the United States will run out of its ability to borrow money on Oct 17, and with only US$30 billion (S$37.4 billion) cash in hand to meet obligations that can run to US$60 billion a day, it will quickly face default.

"I'm telling you that on the 17th, we run out of our ability to borrow, and Congress is playing with fire," he said on CNN's State Of The Union.

"If they don't extend the debt limit, we have a very, very short window of time before those scenarios start to be played out." The US$16.7 trillion US statutory debt ceiling has come increasingly into focus with the two sides deadlocked in a legislative fight originally centered on Republican efforts to defund the Affordable Care Act, Mr Obama's signature health care law.

The US government was forced to shut down on Tuesday for the first time in 17 years after Congress failed to pass a stopgap spending measure to fund government operations.

The House refused to pass a spending bill that did not include the measure to defund Obamacare, and the Democratic-controlled Senate would vote only on a bill stripped of the measure.

In a sign the impasse was expected to continue, the Pentagon on Saturday announced it will recall most of its 400,000 furloughed civilian employees.

Defence contractor Sikorsky, maker of the UH-60 Black Hawk helicopter, said the Pentagon action means it can defer plans to furlough without pay 2,000 of its employees starting Monday.

The absence of Pentagon overseers had been expected to disrupt operations at US defence contractors like Sikorsky.

The House, meanwhile, unanimously passed a bill to retroactively pay the hundreds of thousands of furloughed government workers, in a bid to ease some of the pressure from an unhappy public.

Mr Obama has insisted he will not negotiate with Republicans until they pass a temporary spending bill reopening the government and agree to raise the debt ceiling.

"For as reckless as a government shutdown is, an economic shutdown that comes with default would be dramatically worse," Mr Obama said in a radio message Saturday.

Mr Obama said there were enough Republican and Democratic votes in the House to "end this shutdown immediately." But Mr Boehner blamed the crisis on Mr Obama's refusal to negotiate, and insisted he would not budge without what he described as a "conversation" on spending.

"My goal here is not to have the United States default on their debt. My goal here is to have a serious conversation about those things that are driving the deficit and driving the debt up.

"And the president's refusal to sit down and have a conversation about this is putting our nation at risk of default.

"That's the path we're on," he said, adding that he had hoped the president had cancelled a trip to Asia to initiate a dialogue.

"I decided to stay here in Washington this weekend. He knows what my phone number is. All he has to do is call," he said.

US allies were warily watching the unfolding political crisis in Washington.

At an Asia-Pacific summit on the Indonesian island of Bali, which Obama was supposed to have attended, Singapore Prime Minister Lee Hsien Loong said the US president's absence was "a very big disappointment to us."

"Obviously we prefer a US government that is working to one that is not. And we prefer a US president who is able to travel and fulfill his international duties to one who is preoccupied with national domestic preoccupations," he said.

US Secretary of State John Kerry, who Mr Obama sent to Bali in his stead, warned of the damage to America's diplomatic standing if the shutdown did not end soon.

"The shutdown is not good for business. It's not good for the economy," US Commerce Secretary Penny Pritzker told reporters at the Asia-Pacific Economic Cooperation (Apec) forum.