(Writes through, adds South Korean trade minister)

By Yoo Choonsik and Jack Kim

SEOUL, May 20 (Reuters) - No U.S. beef from animals older than 30 months will enter South Korea, officials said on Tuesday, as they tried to calm safety concerns about the U.S. product and rally support for an embattled free-trade deal with Washington.

Just last month, the U.S. administration heralded a long-desired accord with South Korea that would relax its import rules for beef, seemingly smoothing the way for resumed beef exports to a lucrative market and removing a stubborn obstacle to the U.S. Congress' consideration of the major bilateral trade deal.

But South Korea, the third-largest importer of U.S. beef until the discovery of mad cow disease in the U.S. in 2003, last week delayed quarantine inspections for up to 10 days in the face of public safety concerns.

South Koreans, worried about mad cow disease took to the streets to protest against the deal.

The country's opposition lawmakers said they would block the free trade deal unless the United States agreed to renegotiate the beef pact to address the public's concerns.

"The president stressed that imports of beef from cattle older than 30 months old won't happen in practice, and importers have pledged not to import such products," President Lee Myung-bak's spokesman Lee Dong-kwan said.

The United States said on Monday it was in talks with Seoul to calm safety worries as South Koreans have reacted angrily to the agreement requiring them to accept certain beef cuts that other U.S. trading partners such as Japan still will not import.

In an exchange of letters that South Korean Minister for Trade Kim Jong-hoon said was effective as a formal agreement, U.S. Trade Representative Susan Schwab and Kim agreed that Seoul reserved the right to take "necessary measures" on U.S. beef imports to protect public health.

The United States will apply the same standards on mad cow risk classification to beef intended for export to South Korea and for domestic U.S. consumption, the letters also said.

"Regardless of whether there is a mad cow outbreak, if for some reason our confidence in U.S. quarantine standards is compromised, then we can do it (stop imports)," Kim said.

The level of public anger over the deal, which was meant to be a high point of President Lee's visit to the United States last month, has taken the South Korean government by surprise.

And the opposition United Democratic Party, which has control of parliament until the current session finishes at the end of May, has been riding on the public outcry and stalled on approving the free trade deal with the United States.

"Given public sentiment, it is difficult to talk about the FTA," UDP floor leader Kim Hyo-seuk told reporters at the Seoul Foreign Correspondents' Club. "There must be renegotiations (of the beef pact) or we can't talk about the FTA."

President Lee's conservative party, which won April's general election, will take control of parliament when its new session starts in early June.

But Lee has pushed for the current session to ratify the deal and MPs say it could take weeks and perhaps months for the new session to begin discussions on the trade pact.

South Korea imposed a blanket ban on U.S. beef for about three years after the 2003 mad cow disease outbreak. Before then, it imported around 199,000 tonnes worth $850 million a year. (Additional reporting by Miyoung Kim; Editing by Valerie Lee)

AlertNet news is provided by