SEOUL, May 14 (Reuters) - South Korea is likely to delay Thursday's planned resumption of beef imports from the United States, its agriculture minister said, after mounting public protests over the prospect of meat imports many fear is unsafe.

"We are seriously considering it," Chung Woon-chun said in parliament when asked by a lawmaker if the government was planning to delay the resumption.

South Korea, once the third-largest overseas market for U.S. beef, reached a deal with the United States last month to open its market wider to American beef. South Korea had only permitted boneless beef from cattle under 30 months in age.

South Koreans, worried about mad cow disease, have taken to the streets to protest against the deal. The government has assured the public that U.S. beef is safe and it would halt imports immediately if there was a health risk.

Chung said it would be "physically difficult" to resume quarantine inspections, which would start the flow of U.S. beef back into South Korea.

South Korea imposed a blanket ban on U.S. beef imports for about three years after an outbreak of mad cow disease in the United States in late 2003. Before that ban, it imported around 199,000 tonnes worth $850 million of the product a year.

Trade in U.S. beef has been stalled since last year as Seoul suspended quarantine inspections after prohibited bone chips and spinal material showed up in several U.S. shipments.

South Korea's main opposition party said on Tuesday it would use its last few days in control of parliament to block a sweeping free trade deal with the United States unless it can win changes that would curtail the beef import pact.

It joined with two other left-of-centre parties who said they would file an injunction later on Wednesday seeking to halt the resumption of U.S. beef imports.

The agreement to reopen South Korea's beef market does not require parliamentary approval, but flare-ups could spill over to the South Korea-U.S. free trade pact, which needs to be ratified by parliament in order to take effect. (Reporting by Jack Kim; editing by Jon Herskovitz and John Chalmers)

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