The deaths of the five -- from a group of 10 kidnapped in western Herat province -- were confirmed only hours after a candidate in the September 18 poll from the same area, Haji Abdul Manan, was shot dead as he walked to a mosque to pray.
Manan was the fourth candidate to be killed. The rising toll drew a sharp condemnation from the U.N. mission (UNAMA) in Afghanistan, which is assisting with the election.
These killings constitute violent intimidation of all electoral candidates and their supporters, UNAMA said.
This is unacceptable. UNAMA calls on the Afghan security forces to be on heightened vigilance over the coming weeks, it said, adding those responsible must be brought to justice.
The election will be a litmus test of stability in Afghanistan, where violence is at its worst since the Taliban were ousted in 2001 despite the presence of almost 150,000 foreign troops supporting about 300,000 Afghan security forces.
Poor security, particularly in Taliban strongholds in the south and east, already looms as the biggest challenge to the ballot, along with corruption and fraud.
On Sunday, the bullet-riddled bodies of five members of a group of 10 who had gone missing on Thursday were found dead on a mountainside in Herat, said district chief Nisar Ahmad Popal. It was not clear who killed them, he said.
The campaign workers were supporters of Fawzia Gilani, an outspoken female candidate standing in next month's poll. The other five had turned up unharmed.
GRAFT, POOR GOVERNANCE
Washington officials have said they are worried violence will lead to poor voter turn-outs, especially in the mainly Pashtun south and east where the Taliban are strongest.
The poll, for 249 seats in the lower house of parliament, is also seen as a test of Afghan President Hamid Karzai's credibility after a presidential vote marred by fraud last year.
While not running this time around, Karzai has sought to assert his independence from his Western backers at a time when they are urging him to do more to fight graft and mismanagement.
Such efforts weigh heavily on the minds of U.S. lawmakers and will be an important consideration when President Barack Obama conducts a review of his Afghan war strategy in December.
Obama's administration fears corruption is boosting the Taliban-led insurgency and complicating efforts to strengthen central government control so that U.S. and other foreign forces can begin drawing down from July 2011.
Karzai in turn has not flinched from speaking out against issues which anger Afghans such as civilian casualties and private security firms. Karzai caught Washington by surprise this month when he decreed such firms had four months to disband. Washington's strategy must change, he said.
The strategy of fighting terrorism should be looked at again since the experience of the past eight years shows that war in the villages of Afghanistan is not effective and has only produced civilian deaths, he said in a statement issued by the presidential palace on Sunday.
The rising civilian and military toll also weighs heavily on Obama's mind.
This month, a U.N. report found civilian casualties had risen 31 percent in the first six months of this year, though more than 75 percent of those were caused by insurgents.
But the problem remains very real for foreign troops. The NATO-led International Security Assistance Force (ISAF) said on Sunday an assessment team had found rounds fired by an ISAF helicopter in northern Baghlan province last week may have killed an unspecified number of civilians.
It said the rounds had fallen short because of a faulty gun sight. Afghan officials said last week eight civilians were killed in the strike, which was aimed at insurgents.
Military casualties have also reached record levels this year. ISAF said on Sunday four more American troops had been killed, taking to seven the number killed in the past 48 hours in the south and east.
At least 2,040 foreign troops have been killed since the war began, more than 60 percent of them Americans, according to website www.iCasualties.org and figures compiled by Reuters. Of those, at least 252 have died in the past three months.
Despite being vastly outgunned and outnumbered, the Taliban and its allies still launch bold attacks such as assaults on two foreign bases in eastern Khost province on Saturday.
ISAF said on Sunday the number of insurgents killed in Saturday's attacks had risen to more than 30 from 24, including at least 13 suicide bombers.
(Additional reporting by Sharafuddin Sharafyar in Herat, Sayed Salahuddin, Jonathon Burch, Yousuf Azimy and Andrew Hammond in Kabul; Editing by Jon Hemming)
(For more Reuters coverage of Afghanistan and Pakistan, see: http://www.reuters.com/news/globalcoverage/afghanistanpakistan)