WASHINGTON (Reuters) - The United States and South Korea face enormous challenges in their negotiations on a free trade agreement with only a few months left to find solutions, a South Korean official said on Thursday.

"With so many unresolved issues ... time is not on our side," South Korea"s ambassador to the United States, Lee Tae-Sik, said in remarks at the Heritage Foundation. "Our negotiations are now at a critical stage."

In both countries, certain sectors are strongly resisting any further market openings in the pact. Their demands threaten to derail the talks unless the two governments can keep sight of the "immense" overall gains that would come from reaching a free trade deal, Lee said.

The two sides will hold their fourth round of talks the first week of December in Big Sky, Montana. Negotiators have given up on the idea of reaching a deal by the end of the year, but still need to make significant progress soon to finish in the next three or four months, Lee said.

The Bush administration"s authority to negotiate trade agreements that can not be amended by Congress expires at the end of June. The same law requires the White House to notify Congress 90 days before signing any agreement, meaning it must have a final deal with Seoul by the end of March.

The United States is pressing South Korea in the negotiations to open its rice and other sensitive agriculture markets. It also wants Seoul to increase sales opportunities for U.S. cars and trucks by addressing a number of tariff and non-tariff barriers, including taxes based on engine size.

At the same time, Washington is resisting pressure to dismantle tariffs and restrictive rules-of-origin that protect its textile sector, Lee said. It also does not want to reform anti-dumping laws that South Korea believes have not always been used legitimately, he said.

In another sensitive area for the United States, Seoul wants the free trade pact to include a quota for highly skilled South Korean workers to enter the U.S. market on temporary visas. This is essential for South Korean services companies to take advantage of the increased investment opportunities the pact will provide, Lee said.

Lee was sanguine about recent U.S. elections that gave Democrats control of Congress next year. While Democrats have a reputation for being protectionists, many party members have expressed strong support for a Korea trade pact, he said.