The agreement, struck last month, has been widely criticized as making too many concessions to the United States because it scrapped nearly all the quarantine regulations Seoul imposed earlier to guard against mad cow disease.
Prime Minister Han Seung-soo said in a nationally televised address that Seoul would demand changes to the pact "if a new situation occurs while monitoring negotiations between the United States and other nations."
Han did not elaborate, but the remark apparently meant Seoul would try to improve its beef deal with the U.S. if it is deemed unfair compared with similar agreements Washington might sign with other countries.
"The government will protect the people's heath as the foremost priority in any situation," Han said.
But three main opposition parties agreed later Thursday to seek to dismiss the agriculture minister, pass a resolution calling for a revised pact and launch a parliamentary probe into the past negotiations.
Fueling South Koreans' fears of mad cow disease was a current affairs TV show last week that claimed Koreans are more susceptible than Americans and Europeans to contracting a rare sickness that can result from eating beef infected with mad cow disease.
The government dismissed the claim and repeatedly asserted that American beef is safe, holding hours-long live television debates. But thousands of people have held candlelight vigils in recent days urging the government to scrap the beef deal.
Han reaffirmed Wednesday's pledge by President Lee Myung-bak that the country would immediately halt imports of U.S. beef if a new case of mad cow disease occurs in the U.S.
He also asserted that American beef is safe.
"U.S. beef is consumed not only by Americans, but also people in 96 countries in the world," he said, adding that no mad cow cases have been reported among cattle born after 1997, when Washington strengthened regulations on animal feed.
"Why would the government do things that hurt the health of our people? The government won't make concessions on our people's health in any negotiation with foreign countries," he said.
Seoul suspended imports of U.S. beef after the first U.S. case of mad cow disease appeared in December 2003 in a Canadian-born cow in Washington state.
It resumed limited imports in April last year, but put them on hold again in October when a shipment arrived containing banned animal parts.
Last month's deal to reopen South Korea's market came just hours before President Lee held his first summit with U.S. President George W. Bush. The pact was widely seen as a concession aimed at getting the U.S. Congress to approve a broader trade deal.
Scientists believe mad cow disease, or bovine spongiform encephalopathy, spreads when farmers feed cattle recycled meat and bones from infected animals. In humans, eating meat products contaminated with the illness is linked to variant Creutzfeldt-Jakob disease, a rare and fatal malady.