South Korea's Assistant Agriculture Minister said he was optimistic thefree trade agreement could be reached after the deadline was extended for 48 hours to allow further closed-door negotiations in Seoul.

"There still exists delivery pain but I think we can meet the new deadline for wrapping up the talks," Min Dong-Seok told reporters without detailing the sticking points.

The new deadline will expire at 1:00 am Monday (1600 GMT Sunday, noon Sunday Washington time), officials told a press briefing after a marathon 22-hour negotiating session overnight.

The US Trade Representative's office said the new deadline would still allow any agreement to be reviewed by Congress while President George W. Bush has his authority to "fast-track" deals.

"Negotiations continue between the US and the Republic of Korea on a number of outstanding issues," the office said in a statement.

The free trade agreement, if approved, would be the biggest for the United States since the North American Free Trade Agreement in 1993.

Earlier, the White House warned that negotiators were not making headway.

"The talks aren't going well, and unless the negotiations show some signs of significant progress in the next few hours this agreement will most likely not come together," spokeswoman Dana Perino said.

Trade between the world's largest and 11th largest economies totalled 74 billion dollars in 2006 and studies show this could rise by about 20 percent if trade is liberalised.