"I and my government will acutely reflect on this," he said.
"I and my government will acutely reflect on this"
South Korean president
With the global economy slowing, Lee said he had acted in a hurry and saw the free-trade agreement as a "shortcut" to fulfil his election promise to boost the South's economy.
He said he also saw the move to resume beef imports as part of a broader effort to improve previously strained relations with the US, citing the nuclear threat from neighbouring North Korea.
Anger over the beef deal has triggered a series of street protests over the last month with growing calls for Lee to resign.
The issue has also become a lightening rod for more widespread opposition to Lee, who won a record-breaking landslide in presidential elections last December.
Since taking office earlier this year however, Lee's popularity ratings have plummeted, registering at about 20 per cent in recent days.
Last week South Korea's entire cabinet offered to resign, in an effort to ease some of the pressure.
In his speech on Thursday Lee announced that he would reshuffle his cabinet and top advisors, but gave no details.