South Korean Trade Minister Kim Jong-hoon told reporters there were no easy solutions and he expected a long day of talks. The negotiations were originally scheduled for two days.

Asked if a deal was possible on Thursday, US Trade Representative Ron Kirk said: "We're going to work on it."

South Korea is the United States' seventh-largest trading partner and eighth-largest export market. Last year, the United States exported $28.6 billion worth of goods to South Korea and imported $39.2 billion of products from that country.

The two countries signed a bilateral free-trade agreement on June 30, 2007, but it has been stuck in Congress because of opposition from the US auto industry. South Korean barriers to US beef have also delayed action.

The United States is asking South Korea to renegotiate the auto terms of the agreement, something that is politically difficult for Seoul to do without getting concessions of equal weight from Washington in return.

The United States exported 7,663 cars and light trucks to South Korea in 2009 while it imported 476,857 from automakers there, according to US Commerce Department figures.

Ford Motor Co. and its supporters in Congress complain the lopsided trade is due to South Korean tax and regulatory barriers the Korea-US Free Trade Agreement (KORUS) fails to adequately address.

Senate Finance Committee Chairman Max Baucus also wants Seoul to commit to a process for fully reopening its market to US beef exports, another politically difficult demand because of strong South Korean public opposition.

US officials have declined to say if the United States is willing to make concessions on its side to compensate South Korea for changing the terms of the deal the two countries signed in 2007 after more than a year of negotiation.

The deal would be the second largest US free trade agreement, after the North American Free Trade Agreement with Canada and Mexico. Supporters also see it as linchpin of US economic engagement in the fast-growing Asian region.