South Korean opposition parties called a free trade agreement with the US a "humiliating and treacherous" deal.

The agreement could face some challenges in the South Korean legislature. Nevertheless, the ruling Grand National Party, holding a majority of 172 seats in the 299 seat parliament, welcomed it as a mutually beneficial deal assuring it will work for its ratification.

The details of the FTA were finalised in Washington Friday after a three-year stalemate on auto tariffs.

The renegotiated agreement would allow the US to keep its 2.5 per cent tariff on cars for five years while South Korea would immediately half its current 8 per cent tariff.

South Korea will also ease its safety and environmental standards meeting only US requirements.

Trade Minister Kim Jong-Hoon told reporters Sunday the new agreement is "a by-product that equally reflected each side's interests" but admitted a concession was needed in the disputed auto tariff to help clear the passage of the deal in Washington.

But opposition party lawmakers in Seoul were left fuming at what they said was a "humiliating" agreement with too many concessions for Washington and too little in return for Seoul.

"We have been hit by the North with cannons and now we're being hit by the US with the economy," said Park Jie-Won, floor leader of the Democratic Party, referring to last month's artillery assault by Pyongyang on a border island, which killed four South Koreans.

Jun Byung-Hun, another party leader, called the result "tragic," urging President Lee Myung-Bak to apologise to the public and reopen negotiations.

"We urge him to immediately fire Trade Minister Kim who failed in the negotiations," he said.