SEOUL : South Korean President Lee Myung-Bak insisted Friday he would not re-negotiate a deal on US beef imports that has triggered the biggest crisis yet to face his young administration.

As around 20,000 people protested in central Seoul, Lee said any attempt to carve out a new deal could hurt South Korea even more by jeopardising a free trade agreement with the United States.

But in a sign of the scale of his troubles, his eight most senior aides -- including his spokesman -- offered to resign to take responsibility for the growing beef dispute.

It was not immediately clear if he had accepted their resignations, which were submitted "to assume responsibility for the current situation," spokesman Lee Dong-Kwan told reporters.

Yonhap news agency quoted an unidentified presidential official saying it would clear the way for Lee to reshuffle his team.

His dilemma is that he needs to resume US beef imports to clear the way for Washington to ratify a broader free trade agreement with South Korea.

But there is huge opposition within South Korea to the resumption, rocking his administration just over 100 days in office.

Critics say the government has not secured adequate safeguards against the dangers posed by the human form of mad cow disease.

"If we now demand a re-negotiation, that will cause major problems like a trade dispute," Lee was quoted as saying by his spokesman.

"That may seriously affect exports of our products. By knowing well about such an after-effect, we cannot demand re-negotiation just to get through the current crisis."

Total trade between South Korea and the United States is worth an annual 80 billion dollars, and some studies show this could eventually rise by up to 20 billion dollars under a free trade regime.

Earlier, thousands of riot police barred protesters from marching on the presidential office and residence, manning barricades of buses when the rally neared to just over a kilometre (0.7 miles) from the so-called Blue House.

Some 10,000 riot police have been deployed along roads and sidestreets to bar access. Last week, hundreds of people were arrested and scores injured in clashes when protesters also tried to march on the presidency.

Still, Lee vowed to press on with reforms, while also offering a series of consultations.

"There is no time to hesitate," he said in a speech Thursday morning.

"I will overcome the crisis the country now faces... by pushing ahead with incessant change and reform together with people," he added, citing the impact of the sluggish economy, rising inflation, and the high prices of oil and raw materials.

The presidency said Lee would consult from this weekend with religious and political leaders and academics on the way ahead.

He began Friday by meeting some Buddhist leaders and plans over the weekend to meet senior Christian ministers, his office said.

During next week he will see academics and other political figures, but the presidency did not specify if that would include opposition leaders, who are boycotting the new parliament.

Lee's government has been forced to delay the planned resumption of beef imports and ask Washington not to export beef from cattle more than 30 months old, seen as more vulnerable to possible infection, to ease public anger.

But it has had little apparent effect on damping the calls for him to step down, half a year after sweeping to victory in December presidential elections on a promise to use his business experience to revive the economy.

Expectations of a quick turnaround are fading fast, he has been criticised over a series of key government appointments, and there is stiff opposition to a cross-country canal project he is pushing.

Lee's popularity ratings have tumbled to the low 20s, according to opinion polls, and his conservative Grand National Party suffered big losses in local by-elections Wednesday, its first electoral test since taking office.

- AFP /ls