"The shortfalls that have appeared so far in the beginning of this administration are all my fault."
Lee called on the opposition-dominated National Assembly to "overcome partisan interests" and cooperate in passing the FTA this month. This would be "a major achievement in our political history," he said.
A new parliament dominated by Lee's conservative Grand National Party (GNP) starts on June 2 following its April election victory. But procedural reasons mean it could take months to pass the trade pact.
Seoul wants to act quickly to put moral pressure on the US Congress, which must also ratify the deal.
The current parliament session ends this weekend. But a new four-day session of the existing parliament could be held next week if the parties agree to hold one.
GNP chairman Kang Jae-Sup said he is trying to persuade the opposition to agree on a new session. "We must mobilise everything -- negotiations and pressure," he said.
Lee again promised to halt US beef imports immediately should there be any threat to public health.
The US says its beef is totally safe and refuses to approve the FTA until the beef market reopens.
The main opposition United Democratic Party said Lee's remarks failed to address public fears adequately. It reiterated it would reject the FTA unless the beef deal is renegotiated.
Lee's government sees the trade pact as crucial in putting the economy on a more competitive long-term path. Its traditional industries face cost pressure from China as well as hi-tech competition from Japan.
"Korea is kind of sandwiched between China and Japan," he said Wednesday. "The parliamentary ratification and enforcement of KORUS FTA will help Korea overcome its economic sandwich position."
Total trade between South Korea and the US is worth an annual 80 billion dollars. Some studies show this could eventually rise by up to 20 billion dollars under a free trade regime.