* Fear could stop many Afghans from voting
* Taliban say they plan to disrupt poll
* Washington expects vote to proceed
(Adds U.S. comments in paragraphs 7-8)
By Peter Graff
KABUL, Aug 9 (Reuters) - Insurgent violence and threats have hurt preparation for Afghanistan's Aug. 20 election and could prevent large numbers of terrified Afghans from voting, the United Nations said Sunday.
"Insecurity poses a threat to the ability of a significant number of Afghan to exercise their right to vote," a U.N. report, issued less than two weeks before the poll.
"It is not surprising that the fear as well as the covert and overt intimidation that characterizes the insurgency, creates an underlying reticence to stand for office and to campaign or to vote," the report said.
Taliban militants say they plan to disrupt the poll, which U.S. President Barack Obama has identified as the most important test of Afghanistan's political progress this year. Obama has rushed thousands of extra troops to the country to help secure the vote.
President Hamid Karzai, who has lined up the backing of many of Afghanistan's powerful regional chiefs, is seen as likely to win re-election against a fractured opposition.
Diplomats worry that sustained violence on polling day, threats that sharply reduce turnout or allegations of large-scale fraud could make it hard to present any result as legitimate, worsening the instability in the country.
Senior U.S. officials said on Sunday they expected the election to proceed as scheduled and would work with Afghan officials to create a secure environment for voters.
"The Afghan people are ready and waiting, and our aim is to ensure that there's a level playing field, that the Afghan people have an opportunity to freely choose their next leader in security," Susan Rice, the U.S. ambassador to the United Nations, said on CNN's "State of the Union."
Violence in Afghanistan has reached its highest levels since the Taliban were driven from power in 2001, especially in the south and east of the country.
In the most violent areas, far fewer people are attending political rallies than in the past, for fear of attack, the U.N. report said. In some provinces -- such as Ghazni in the south -- candidates are not campaigning at all, only sending posters.
The report identified nine people killed in suspected election-related attacks, including four staff for Karzai's campaign, as well as numerous failed assassination attempts.
Insurgents had broadcast threats by radio in two provinces and had distributed pamphlets with threats in two others. It drew particular attention to the risks faced by female candidates and voters. The Taliban and other groups violently oppose women's participation in politics.
Female candidates make up 10 percent of those registered to stand for provincial council seats across the country, but some have been subjected to death threats. The house of one was fired on after letters were plastered on buildings warning her to stop campaigning, the report said.
In some areas, men may cast votes on women's behalf -- a practice the report said is seen as culturally acceptable in conservative parts of Afghanistan, despite being illegal.
Suspiciously large numbers of women's registration cards have been issued in some areas. In one case, the report said, a local official was accused of stealing 9,000 voter registration cards intended for women, "and to have gone to seven local political allies with the intention of using the cards fraudulently."
The incident, it said, is under investigation. (Additional reporting by Paul Simao and Susan Cornwell in Washington; Editing by Charles Dick)