President Barack Obama and US politicians have pushed separate plans to avert an unprecedented default on America's national debt, with no signs of an immediate breakthrough.

Mr Obama called on politicians to approve a giant package that would not only prevent a default by raising the government's borrowing limit, but also slash trillions of dollars from the country's enormous deficit.

He challenged politicians "to do something big".

But opposition Republicans, who control the House of Representatives, reject Mr Obama's proposal to raise some taxes in addition to cutting spending.

They plan to vote next week on legislation that would tie an increase in the debt limit to a constitutional amendment requiring a balanced budget. Such an amendment is unlikely to get enacted.

Offering more promise is a bipartisan measure being worked on in the Democratic-led Senate that would allow Mr Obama to raise the debt limit without a prior vote by politicians. That plan would likely include spending limits.

Even as he pushed for a more sweeping measure, Mr Obama did not rule out the Senate's more modest plan.

"We are obviously running out of time," he said at White House news conference. It was his second news conference of the week, a testament to the overriding political and economic significance of the issue that has convulsed Washington this summer.

Numerous officials have cautioned that a default will occur if the debt limit is not increased by August 2, warning also of a calamitous effect on an economy struggling to recover from the worst recession in decades.