South Korean officials played down chances of an immediate deal as the United States stood by its calls for greater market access in South Korea's heavily protected agriculture sector.

The pact, if agreed, would be the biggest US free-trade deal since the North American Free Trade Agreement in 1994.

Scores of activists opposed to the deal exchanged kicks and punches with police who outnumbered protesters as they blocked a march towards a Seoul hotel where the five-day talks got underway.

Police briefly detained and then released some of the protesters during the scuffles. No serious injuries were reported.

"No South Korea-US FTA!" demonstrators chanted in a sit-in under drizzle in front of the hotel, which was guarded by some 3,000 riot police. Some 600 protesters later held another rally at the spot demanding the talks be stopped.

Activists have pledged daily demonstrations outside the hotel and in other downtown areas, saying that a free trade accord would hurt their livelihoods.

The world's largest and Asia's fourth largest economies have set a goal of striking an FTA in March next year. But expectations for an immediate breakthrough were low.

"Our government's basic stance is that if the US requests are unacceptable, no settlement will be made," South Korea's chief negotiator Kim Jong-Hoon told reporters before the talks began.

"If our people don't accept the free trade deal with the US, the National Assembly won't ratify the accord."