The United States and South Korea hope to make "significant progress" in crucial talks launched to frame a free-trade agreement ahead of a looming deadline, a top US negotiator said.

The two allies are racing to conclude what could be the biggest US trade deal in 14 years by the end of March to give the US Congress its requisite three months to consider the deal.

"We are rolling up our sleeves and we are hoping that we can come out of this with significant progress that will lead to a final deal over the course of the next six to seven weeks," US chief negotiator Wendy Cutler told reporters as the seventh round of talks was launched in Washington.

Some 200 negotiators from each side will this week try to resolve outstanding differences, particularly on three sensitive issues -- US anti-dumping rules, and non-tariff barriers in South Korea's auto and drug markets.

The United States and its key Asian military ally South Korea started talks in June aiming to marry their economies with a combined trade of more than 70 billion dollars.

"We certainly have come a long way since our first negotiating round in Washington last June but clearly this is the week to quicken our pace, focus and do everything we can do to conclude this deal," Cutler said.

"However, let me be clear that no one should expect a final agreement to be announced this week," she said.

At least one more round of talks is expected to firm up any agreement, negotiators said.

If they bag the deal, it would be the biggest free-trade agreement since the 1993 North American Free Trade Agreement linking the United States, Canada and Mexico.

As the negotiations moved full steam ahead, US lawmakers have threatened to withhold support for any free-trade deal after South Korea refused to budge in opening up its market to US beef imports.

Seoul last year lifted a three-year ban on US beef imposed to keep out mad-cow disease and agreed to accept shipments of boneless beef. But it has since blocked all three shipments sent after finding tiny bone fragments in them.

The US side claims South Korea is using the fragments as a pretext to exclude its beef and to protect local farmers.

Still, experts say there is political will by both governments and widespread support among US lawmakers and key US industries to conclude an agreement.

Brian Peck, a former senior director at the US Trade Representative (USTR) office, said there is a door open still, "as long as it (the beef issue) is resolved by the time Congress has to consider the FTA."

"It is a challenge but it is doable," said Peck, now counsel for international trade with US legal firm Crowell and Moring LLP.

US President George W. Bush's powers to fast-track trade pacts expire in June, binding US negotiators to present a deal for a mandatory 90-day congressional review for an outright yes-or-no vote without amendments.

"While I am hopeful that we can make substantial progress this week, additional work will be required at various levels to conclude a comprehensive and balanced agreement," Cutler said.

Reports indicate that Seoul could address US demands to open South Korea's market wider to US vehicles and pharmaceuticals if Washington eased anti-dumping rules against Korean goods.

South Korean officials have long complained that anti-dumping duties have been used as a means to block shipments of semiconductors and other high-tech South Korean products.

Cutler said that the issues of increased access to South Korea's automotive and pharmaceutical markets remain "a top priority" for the United States, adding she saw a "good sign" of progress.

"We have made it very clear to the Koreans that the two issues need to be satisfactorily addressed for any FTA that is concluded," she added.