The US government shutdown entered its second week with ominous signs that the country was edging closer to its first debt default in history.

Republican House Speaker John Boehner ruled out any measure to boost borrowing authority without concessions from President Barack Obama.

Treasury Secretary Jack Lew warned that budget brinkmanship was "playing with fire" and implored Congress to legislate to reopen the government and increase the nation's $16.7 trillion (£10.4trn) debt limit.

He reiterated that Mr Obama has no intention to link either Bill to Republican demands for changes to the three-year-old health-care law and spending cuts.

But a defiant Mr Boehner insisted that the President must negotiate if he wants to end the shutdown and avert a default that could trigger a financial crisis and recession that would echo 2008 or worse.

"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 insisted.

Mr Boehner also warned that Democrats lack the votes to pass a continuing resolution that would keep the government operating.

The shutdown has pushed millions of workers off the job, closed national parks and museums and stopped an array of government services.

However, a significant number of the laid-off federal employees headed back to work today.

Defence Secretary Chuck Hagel ordered nearly 350,000 back to the job.

The decision was based on the Pentagon's interpretation of the recent Pay Our Military Act, which targets welfare-related civilian staff.