US President Barack Obama may call a weekend debt-limit summit with lawmakers at Camp David, best known as the site of hard-fought Middle East peace talks, a Democratic aide said Thursday. "I can confirm it's being discussed as an option. Nothing confirmed," the aide told AFP amid growing unease about prospects the White House and its Republican foes will work out a deal to avert an early-August debt default sure to spook global markets. The president was preparing for more tough talks Thursday to avert an August debt default, after Moody's held out the threat of downgrading the sterling US credit rating. Obama was to hold a fifth straight day of White House negotiations with Republicans and fellow Democrats at 4:15 pm (2015 GMT), and planned to take stock of the apparent stalemate on Friday, a Democratic aide said. "Friday is not a hard deadline," an aide said after contentious talks on Wednesday, but "the clock is ticking, they have to get this done." The US debt ceiling now stands at $14.29 trillion and the budget deficit is expected to hit $1.6 trillion this year. The US hit the ceiling on May 16 and has used spending and accounting adjustments, as well as higher-than-expected tax receipts, to continue operating without impact on government obligations. But by August 2, the government will have to begin withholding payments to bond holders, civil servants, retirees or government contractors. Moody's credit ratings agency sparked worldwide concern Wednesday when it placed Washington's triple-A debt rating on a downgrade watch because of rising prospects that the bitterly divided US leaders would fail to strike a deal. China also expressed concern after its Dagong credit ratings agency echoed Moody's by putting US sovereign debt on downgrade watch, citing weak US economic growth and the likelihood that fiscal deficits would remain high.