South Korea's president made a rare trip to parliament on Tuesday to urge legislators to vote through a free trade agreement (FTA) with the United States which critics say could threaten South Korea's national interests.

The FTA, which some studies say could boost the $67 billion two-way trade between the allies by as much as a quarter, was approved by the U.S. Congress last month and signed into law by President Barack Obama.

The deal is the biggest U.S. trade pact since the North America Free Trade Agreement went into force in 1994.

It had been expected to sail through South Korea's parliament where the ruling Grand National Party (GNP) has a majority.

But opposition party members have been trying to block it with some complaining that the deal gives U.S. automakers a major inroad into the South Korean market.

South Korean farmers are also worried about a flood of U.S. farm products and many joined a protest of about 20,000 people in the capital Seoul on Sunday to oppose the pact.

Opposition members also say a provision in the deal that gives foreign investors the right to have international arbitrations hear a dispute in South Korea is akin to South Korea giving up its sovereignty.

President Lee Myung-bak told legislators he would seek a renegotiation of that provision, known as the investor state dispute (ISD), if they passed the bill.

"If parliament approves ratification of the FTA bill and advises both the United States and South Korea to renegotiate on the ISD ... then renegotiation will be requested of the United States within three months of the bill taking effect," Lee said.

The ruling party stresses the FTA is vital for Asia's fourth largest economy.

The GNP has a comfortable majority in parliament but it has been unwilling to risk political damage by pushing the bill through ahead of general and presidential elections next year.

The main opposition Democratic Party said it would consider Lee's offer although other opposition legislators said his offer was not enough.

Tuesday's visit was only the third time Lee had been to the legislature since he took office in 2008.

The GNP has criticised the Democratic Party for trying to block a deal that was negotiated and signed when it was in power in 2007.

Democratic Party leader Sohn Hak-kyu had pledged opposition parties would not back down until the bill was revised to fix what opponents see as compromises of national interests introduced last year to address U.S. automakers' concern.

(Additional reporting by Christine Kim, Jumin Park and Yoo Choonsik; Editing by Robert Birsel)