Some 15,000 troops will guard against possible violent protests as negotiators, now meeting for a sixth time, try to forge agreements ahead of the next round of talks in February.

Key sticking points have been left off the agenda as the two sides spend five days trying to strike bargains on less important issues.

Major disagreements over US anti-dumping and countervailing rules, and South Korean non-tariff barriers to the auto and drug markets, will be left on one side, the foreign ministry said in a statement.

"Excluding core issues of contention from negotiations at this round, the government will focus on trying to agree on other issues," the statement said.

But time is short, with Washington needing an agreement no later than early April before President George W. Bush's powers to fast-track free-trade deals expire in June.

A free-trade agreement (FTA) with South Korea would be the United States' biggest deal since the North American NAFTA pact in 1994, marrying two economies whose bilateral trade in 2005 was 72 billion dollars.

South Korea, dependent on trade for 70 percent of its gross domestic product, estimates the pact can raise its GDP by two percentage points.

But talks have been difficult with a long list of demands on both sides.

The United States wants to get rid of South Korean tariffs and other market-access restrictions on US services and goods, including autos and pharmaceuticals.