SEOUL, May 8 (Reuters) - South Korea's prime minister on Thursday left the door open to revising a U.S. beef import deal as criticism mounted at home that Seoul had agreed to the pact to please Washington and ignored safety concerns.

South Korean officials had previously insisted the agreement could not be renegotiated.

"We will keep watch on the negotiations between the U.S. and other countries and will at any time ask to revise the (beef) agreement if any new situation comes about," Prime Minister Han Seung-soo said at a news conference.

South Korea agreed last month to open its market wider to American beef after U.S. lawmakers said a separate, sweeping bilateral free trade deal would not make it through Congress until Seoul made concessions on beef.

On May 15, South Korea resumes quarantine inspection on all cuts of U.S. beef from animals of any age, which will start imports flowing again and lifts a blanket ban Seoul had imposed in 2003 following an outbreak of mad cow disease.

South Koreans have taken to the streets to protest as rumours have spread quickly in the world's most wired country making claims such as diapers and cosmetics posing a risk for mad cow disease because beef products are used in their production.

One local TV station claimed Koreans carry a special gene that makes them more susceptible to mad cow disease.

The government, which says U.S. beef is safe, has brought out scientists to knock down some of the claims while leading newspapers have denounced the rumours as fear mongering.

"Koreans have ignored scientific evidence and refused to communicate in a rational way," the major daily Dong-A Ilbo said in an editorial on Thursday.

But the outrage over the beef deal has caused the support rate for new President Lee Myung-bak, who took office in February, to plummet.

Lee said on Wednesday he would immediately suspend imports if U.S. beef posed a health risk to the South Korean public.

U.S. Deputy Secretary of State John Negroponte tried to ease concerns during a trip to Seoul.

"I would very much hope that as the Korean government and people discuss and consider this issue, that the discussions be based on scientific facts and not on imagined problems," he told reporters on Wednesday.

At present, South Korea, once the third-largest import market for U.S. beef, admits only boneless beef from cattle younger than 30 months old.

But even that trade was stalled since last year when Seoul suspended inspection after prohibited bone chips showed up in several U.S. shipments.