KABUL (Reuters) -- Eight U.S. troops were killed in bomb attacks in southern Afghanistan, the alliance has said, in one of the deadliest weeks for U.S. troops ahead of a presidential runoff.
Several troops were also wounded in "multiple, complex [bomb] attacks," just a day after 11 U.S. troops died in separate helicopter crashes.
The mounting violence comes at a time when U.S. President Barack Obama is weighing up his options on whether to send additional troops to Afghanistan to fight a Taliban insurgency. that is at its strongest since 2001.
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 October 27, Karzai's camp said a runoff vote in the presidential election must take place even if his challenger, Abdullah Abdullah, quits the race.
Karzai agreed to a runoff under severe international pressure last week after a UN-led fraud investigation annulled a huge chunk of his votes in the original August 20 election and triggered a second round.
Fuelling talk that he might pull out from the race altogether, Abdullah set out a range of conditions this week ahead of the November 7 second round that were immediately rejected by Karzai's team.
Wahid 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. "This is a legal process and should go ahead.... 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."
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.
Abdullah has given Karzai until October 31 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.
"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," the 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."
The Taliban have already vowed to disrupt the November 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.
A public opinion poll conducted by the Asia Foundation in Afghanistan in June-July, however, found that more Afghans were optimistic about their country compared to an earlier survey.
In a poll funded by the U.S. Agency for International Development, the foundation said 42 percent of respondents thought Afghanistan was "headed in the right direction" compared with 38 percent recorded in 2008.
In Luxembourg, the European Union said it would increase aid to Afghanistan but warned the situation was deteriorating and reforms were almost nonexistent in some areas.
In an appeal made at a time when public support for the war in Afghanistan is fading in Western countries, Russia, China, and India urged the world to remain engaged in Afghanistan to help it counter extremism and drug trafficking.