KABUL - Eight U.S. troops were killed in bomb attacks in southern Afghanistan on Tuesday ahead of a run-off presidential election, the NATO-led alliance said, in the deadliest month for U.S. troops since the start of the war eight years ago.

The mounting violence comes at a time when U.S. President Barack Obama is weighing up his options on whether to send more troops to Afghanistan to fight a Taliban insurgency that is at its fiercest since 2001.

NATO-led forces said several troops were also wounded in multiple complex (bomb) attacks in the south, just a day after 11 U.S. troops died in separate helicopter crashes in Afghanistan..

An Afghan civilian was killed and several service members were also wounded in these incidents, it said. No other details were immediately available.

Separately, the NATO force said it had recovered the remains of three civilian crew members and the wreckage of a plane missing since October 13 in the remote Nuristan province. It said hostile action was not believed to be the cause of the crash.

U.S-led efforts to stabilize the country have been further complicated by weeks of political tension over a presidential election marred by widespread fraud in favor of incumbent Hamid Karzai, forcing a second round set for November 7.

On Tuesday, Karzai's camp said a run-off vote in the presidential election must take place even if his challenger Abdullah Abdullah quits the race.

Karzai agreed to a run-off under severe international pressure last week after a U.N.-led fraud investigation annulled a huge chunk of his votes in the original August 20 election. Fuelling talk that he might pull out from the race altogether, Abdullah set out a range of conditions this week ahead of the vote. Karzai immediately rejected the demands.

Waheed Omar, Karzai's chief campaign spokesman, told Reuters the election must take place even if Abdullah quits.

We should not deprive the people from their right of voting and their right of citizenship, he said. Whether or not president and Abdullah take part in the run-off or not should not result in depriving the people from what they want.

DEAL?

Abdullah has given Karzai until Saturday to sack the country's top election official and meet a range of other demands but would not say what he would do if his conditions were not met. He could not be reached for comment on Tuesday.

But concerns about security and a repeat of the fraud that tainted the first round have cast a shadow over the process, and made some diplomats suggest that a power-sharing deal between the two contenders looked more practical.

Karzai and Abdullah have so far publicly denied suggestions they could be in talks on a possible power-sharing deal and said that holding the second round was key to strengthening democracy.

The Taliban have already vowed to disrupt the poll, highlighting the kind of challenges that face Western powers seeking to turn the tide in the eight-year war.

The protracted process and the prospect of another election has disillusioned many voters, and the onset of the bitter Afghan winter has also created additional challenges.

Widespread fraud in August 20 presidential and provincial council polls has deeply undermined the credibility of Hamid Karzai's government, the main beneficiary of the rigging, International Crisis Group said in a statement.

A flawed second round will hand Taliban insurgents a significant strategic victory and erode public confidence in the electoral process and the international commitment to the country's democratic institutions.

(Writing by Maria Golovnina, additional reporting by Golnar Motevalli and Sayed Salahuddin; Editing by Sanjeev Miglani)

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