South Korea's liberals will use their last few days in control of parliament to block a sweeping trade deal with the United States unless they can win changes on a separate US beef import pact, its leader said.

Lawmakers from liberal parties, which fall to minority status when a new parliament is seated in late May, have called on South Korea to either scrap or restrict a deal Seoul reached with Washington last month to open the market wider to US beef imports.

"We want to make this very clear: We must start renegotiations," the head of the United Democratic Party Sohn Hak-kyu said in a party leaders' meeting.

Sohn, whose party suffered a crushing defeat to conservatives in an April 9 parliamentary election, added if a deal is reached on beef, the National Assembly can then take up the free trade agreement with the United States.

South Koreans, worried about mad cow disease, have taken to the streets to protest the beef deal. The South Korean government has assured the public that US beef is safe and it will halt imports immediately if there is a health risk.

The government of new South Korean President Lee Myung-bak will likely have to wait until at least June to see the free trade deal approved in the new parliament where its conservative allies hold the majority, analysts said.

"It will take a very long time for the next session to ratify the agreement and make legal amendments," Minister for Trade Kim Jong-hoon told parliament.

US lawmakers had said the deal, that studies indicate could boost their $78 billion in annual, two-way trade by about $20 billion, would not win congressional approval unless South Korea opened its market fully to US beef imports.

South Korea, which had imposed a blanket ban for about three years on US beef after an outbreak of mad cow disease in the United States in late 2003, struck the beef deal last month when Lee was about to hold his first summit with George W. Bush.

South Korean officials have said ratification of the free trade deal by its parliament would put pressure on the US Congress to vote on the deal before the November presidential and congressional elections. (Reuters)