WASHINGTON - US President Barack Obama and top lawmakers searched for an elusive deal on Sunday necessary to avert an unprecedented debt default by the United States that economists warn could have a catastrophic ripple effect on financial markets.

Behind closed doors at the White House and in the marbled halls of Congress, polarized US leaders struggled to come together despite fears that the stalemate could send global markets tumbling, starting with early Monday trade in Asia.

Democratic Senate Majority Leader Harry Reid accused Republicans on Saturday of bringing the world's richest country to "the brink of default" by rejecting a deal that would last through the 2012 US election.

"Anything less than that will fail to provide the certainty that the markets - and the world - are looking for, risking an immediate downgrade of America's credit rating," he said.

"Now is the time for cooperation."

Earlier, Republican House Speaker John Boehner told members of his majority on a conference call that he hoped for a deal within 24 hours to soothe investor worry, notably in Asia, a participant told AFP.

"We are working, and I'm confident there will be resolution. There has to be," Boehner told the group, promising "real cuts" in spending that help put Washington on a "sustainable" path, the source said.

The speaker said the hard-fought negotiations centered on a two-step process to cut $3-4 trillion in spending over 10 years as part of a deal to raise the $14.3 trillion US debt limit by an Aug 2 deadline, the participant added.

Agence France-Presse