US and South Korean negotiators have failed to settle a dispute over beef imports which threatens to scupper a wider free trade deal, Seoul has admitted.

The two countries could not narrow differences over the presence of bone fragments in beef shipments, said chief South Korean negotiator Lee Sang-Kil after two days of talks.

"We cannot say the talks broke down as we agreed to continue talks. But we have not yet agreed on the date for the next round," he said.

South Korea, once the third-largest market for US beef, last year lifted a three-year ban imposed to keep out mad-cow disease and agreed to accept boneless shipments.

But quarantine inspectors blocked all three shipments totalling 22 tons which were sent since then after finding tiny bone fragments in them.

The US side says Seoul is using the fragments as a pretext to exclude its beef and to protect local farmers. US Agriculture Secretary Mike Johanns has previously harshly criticized South Korea for banning the shipments for what he called "invented" reasons.

US beef exports to Korea were worth more than 820 million dollars in 2003.

In Washington, seven US Senators warned Thursday that South Korea's persistent refusal to accept US beef imports threatens Congressional support for the free trade agreement (FTA).

The seven, from beef-producing statesm gave the warning to Seoul's ambassador in Washington Lee Tae-Sik.

"Failure to open your market to this safe product will make it difficult for Congress to pass a final KORUS FTA (South Korea-US Free Trade Agreement)," they said in the letter.

A seventh round of FTA negotiations starts Sunday in the United States. A deal must be reached no later than early April if it is to be cleared by Congress before President George W. Bush's powers to fast-track trade agreements expires in June.

After the sixth round in Seoul, negotiators admitted differences on three sensitive issues -- US anti-dumping rules, and non-tariff barriers in South Korea's auto and drug markets.

An FTA with South Korea would be the United States' biggest deal since the North American NAFTA pact in 1994, marrying two economies whose bilateral trade in 2005 was worth 72 billion dollars.

South Korean officials say the FTA will help Asia's third-largest economy increase exports and upgrade its industrial structure. The government is pushing for a pact despite hostility from farmers and other workers who fear for their jobs.