WASHINGTON (Reuters) - The United States believes Zimbabwe's recent election was flawed and it doesn't plan to loosen sanctions against President Robert Mugabe's government until there are signs of change in the country, the State Department said on Monday, despite an endorsement of the vote by Southern African leaders.

Mugabe, who has ruled Zimbabwe since independence from Britain in 1980, is set to be sworn in as president as early as this week, extending his 33-year rule of the country after winning the July 31 election.

The 15-nation Southern African Development Community, which helped broker a power-sharing deal after disputed elections in Zimbabwe in 2008, backed Mugabe's re-election on Sunday.

"The United States stands by our assessment that these elections, while relatively peaceful, did not represent a credible expression of the will of the Zimbabwean people due to serious flaws throughout the electoral process," said State Department spokeswoman Jen Psaki.

"We have made clear to the government of Zimbabwe and the region that a change in U.S. sanctions policy will occur only in the context of credible, transparent and peaceful reforms that reflect the will of the Zimbabwean people."

The opposition Movement for Democratic Change, led by Morgan Tsvangirai, on Friday dropped its court challenge to Mugabe's landslide win, saying it doubted it would get a fair hearing.

The Zimbabwe Election Support Network said registration flaws may have disenfranchised up to a million people out of 6.4 million registered voters.

The United States imposed sanctions on Mugabe in 2003. The sanctions, which ban more than 250 Zimbabwean individuals and companies from doing business with the United States, were extended in 2009.

Mugabe has ruled Zimbabwe since it gained independence from Britain in 1980 and is Africa's oldest leader.

Psaki said it was "feasible" that the U.S. could review sanctions toward Zimbabwe if conditions in the country improved, but for now they would remain.

Soon after his nomination as Secretary of State, John Kerry wrote to Mugabe outlining the U.S. position on elections and the opportunities it provided for the country.

The U.S. had said it was willing to roll back sanctions and expand trade and investment if elections were conducted in a free and credible environment.

(Reporting by Lesley Wroughton; Editing by Christopher Wilson)