WASHINGTON, May 7 (Reuters) - The Bush administration wants to finish the Doha round of world trade talks before it leaves office in January 2009, but advanced developing countries like Brazil, China and India must open their markets as part of the deal, the top U.S. trade official said on Wednesday.

"It is our objective to see the Doha round conclude on our watch," U.S. Trade Representative Susan Schwab said in a speech at the State Department. "That can't be just any old Doha round. We're not going to sign up to a bad agreement just to get an agreement in 2008."

Schwab told the Council of Americas, a Western Hemisphere business group, she saw a window of opportunity for countries to reach a deal this year and failing to do that could delay an agreement indefinitely.

"If you don't do it in 2008, 2009 and 2010 are not likely to help us accomplish it" because of the change in the U.S. administration, a turnover in the leadership of the European Commission and a variety of other reasons, Schwab said.

The United States, Brazil, the European Union and a core group of other countries were working hard in Geneva to try to narrow the gaps in the negotiations, Schwab said.

But she indicated other countries were holding back in talks, which are aimed at spurring economic development through increased trade in agriculture, manufacturing and services.

"The question is what is the contribution that China is going to make? What is the contribution that India is going to make?," said Schwab, who was due to meet with Indian Commerce and Industry Minister Kamal Nath on Thursday in New York.

Agriculture has been at the core of the negotiations since they began in November 2001 in the capital city of Qatar.

For a deal to come together, rich countries such as the United States and those of the European Union need to "really lean forward and make the tough decisions to make the tariff cuts and the subsidy cuts that we need to do," Schwab said.

But it also requires "this tier of advanced developing countries taking on a set of (market-opening) responsibilities that are more than other developing countries," she said.

Negotiators have been working toward a possible ministerial meeting in Geneva in late May, although there have been signs that could slip to June or July.

Schwab said countries must act quickly to reach the "elusive breakthrough" needed to bring the talks to a close by the end of the year.

"The next four to six weeks are really going to be critical. I know I've said it before, but I mean it," Schwab said.

She also suggested the payoff from a Doha round deal would be less immediate than bilateral free-trade agreements, which eliminate many tariffs the first year they go into effect.

"When we're talking about the Doha round, we're talking about trade in 2015, 2020 and 2025," Schwab said.