US Trade Representative Susan Schwab and her South Korean counterpart Kim Hyun-chong inked the deal on Capitol Hill, where mostly Democratic lawmakers have vowed to vote against it unless it is amended to address South Korea's non-tariff barriers, especially in the automotive industry.

But Schwab made it clear Saturday that the signed FTA "will stand on its own, without amendment," saying the Democratic-controlled Congress "will come to understand the details and learn just how compelling a deal it is."

"We must not fall back," Kim said, calling it a comprehensive agreement with "enormous benefits" for both sides.

The agreement was signed just hours before President George W. Bush's "fast track" trade authority expires at midnight Saturday.

"Fast track" authority gives the president unbridled powers to broker trade agreements, which are then subject to an up or down vote by Congress without any amendments possible.

Bush in a statement called on Congress to ratify the agreement, saying it would bring "considerable benefit" to Americans and boost the US-South Korea partnership, which he said had served as a force for stability and prosperity in Asia.

"It will generate export opportunities for US farmers, ranchers, manufacturers, and service suppliers, promote economic growth and the creation of better paying jobs in the US, and help American consumers save money while offering them greater choices," he said.

The agreement is the biggest free trade deal since the 1993 North American Free Trade Agreement (NAFTA), and the two sides concluded negotiations on the pact on April 1 after 10 months of talks.

"The United States and Korea have shown the world that two advanced, industrial countries were able to agree on a gold standard FTA," Schwab said. "What is more impressive is that we did it in only one year."

Admitting that the negotiations that led to the signing had been controversial, Schwab cited a Korean proverb "A turtle only travels when it sticks its neck out.

"Both negotiating teams stuck their necks out and traveled far," she said.

Democratic leaders, including House of Representatives Speaker Nancy Pelosi and top presidential contender Hillary Clinton, as well as several lawmakers from Bush's Republican party are against the pact.

They are worried it would increase the already large US trade deficit, cost US jobs and make the United States less competitive.

Many are particularly concerned over the automobile provisions in the agreement, which they said did not go far enough in dismantling the non-tariff barriers in South Korea. They also want Seoul to fully open up its beef market.

Last year, for example, South Korea exported more than 700,000 cars into the United States while the United States exported fewer than 5,000.

"We cannot support the (agreement) as currently negotiated," Pelosi and four powerful lawmakers from her Democratic party said in a statement on the eve of the signing.

An amendment seeking stronger labor and environmental standards was included in the pact at the last minute after the Bush administration and Congress reached a bipartisan deal in May to include such provisions in all trade pacts.

US-South Korea goods trade was valued at 78 billion dollars last year and studies indicate that the FTA will add anywhere from 17 to 44 billion dollars a year to the US economy.

Washington believes if the agreement is approved, it could trigger a wave of trade liberalization and economic reform throughout Asia, where it has such pacts only with Singapore and Australia.