United States and South Korean negotiators on Sunday struggled to seal a major trade and property for sale in South Korea deal with a deadline just hours away and neither side daring to predict the outcome.

They face a 4pm GMT deadline -- noon in Washington and 1am on Monday in Seoul -- to reach what would be the largest US trade pact in 15 years.

"This is the day when both sides put their bottom line on the table ... we're down to the wire," US Trade Representative spokesperson Stephen Norton said in Seoul where the final round of talks have been taking place for the past week.

But he added: "There's still a very willing spirit [to conclude a deal]."

One protester against the proposed free-trade agreement set fire to himself near the central Seoul hotel where the talks have been going on.

A hospital official said the man, identified only by his family name Huh, was in serious condition with third-degree burns.

South Korean officials have said the most contentious issues were agriculture, including beef and oranges, autos and textiles.

"We're doing our best until the very end," a South Korean negotiator said during a break from the talks. "But we don't know if this will be the end or not." He declined to elaborate.

In the past few days, US leaders have been loudly pressing their demands that South Korea open its tightly protected market to beef and vehicles.

South Korea used to be a major market for US beef until it was banned over health concerns. The issue is not strictly part of the negotiations but has become closely linked.

Some estimates say an agreement could add $20-billion to the already more than $70 billion two-way trade each year.