The New York-based International Rescue Committee (IRC) confirmed five Afghan staff members, all men aged in their 20s, were taken hostage on Sunday and killed by their captors on Monday.
A sixth man killed in the western province of Herat was a local representative of the Afghan ministry of rural rehabilitation and development, who was travelling with the IRC team.
"The taxi they were travelling in was stopped by armed men and forced to drive to another location. On Monday evening, the captors delivered their bodies to a local hospital," the IRC said in a statement.
"The IRC has no further information about the incident or any motivation behind the killings."
Separately, six truck drivers being held hostage in the eastern province of Paktia were also found dead on Monday evening.
The spate of killings were reported as President Hamid Karzai held a second day of talks in Pakistan, calling on Islamabad to help set up peace talks to end 12 years of conflict in Afghanistan.
Karzai strongly condemned the murders, which he blamed on Taliban insurgents, and he made a veiled reference to the widespread belief that elements in Pakistan support these militants.
"The killings show the Taliban and their foreign masters want Afghanistan to remain a dependent and poor country forever," the president said in a statement.
The IRC workers were returning after holidays to Herat's Gulran district to continue work with the National Solidarity Programme, which helps rural communities carry out development projects.
The NSP, created in 2003, makes grants to village councils to fund roads, bridges and other infrastructure schemes as well as running education and literacy programmes.
"The IRC is devastated and grief-stricken by the deaths of our colleagues who all were working to make a better Afghanistan," said IRC president George Rupp.
"We offer our deepest and heartfelt condolences to the families, friends and co-workers of all those who were killed."
The IRC, which has worked in Afghanistan since 1988, said it had temporarily suspended all operations in the country.
Gulran is a remote district bordering on Iran and Turkmenistan, but Herat is one of the most stable and safe provinces in Afghanistan.
The Taliban, who ruled Kabul from 1996-2001, regularly target government employees in their nationwide guerrilla war against the Western-backed administration in Kabul.
Sections of the Pakistani state have been widely accused of funding and controlling these rebels for years, but the Islamabad government denies all allegations and says it will work to stop the war in Afghanistan.
Abdul Wali Sahee, deputy governor of Paktia province, which borders on Pakistan, said the six truck drivers found dead on Monday were abducted by Taliban two weeks ago on their way to Khost province.
"We tried to negotiate their release with the help of village elders, but Taliban killed them. We found their dead bodies last night," he said.
The militants made no immediate comments on Karzai's visit to Pakistan or the latest killings.
In a separate violent incident one suicide bomber was shot dead by security guards in Kabul on Tuesday evening when he tried to attack a government ministry. One civilian was injured.
Pressure is increasing on Afghan and international leaders to negotiate a peace deal with the Taliban to foster stability before NATO-led combat troops depart by the end of next year.
COMMENTS (4)
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@Realist:
If they come to dislodge religious fundamentalists like the taliban, i'd welcome any help with open arms. I would hate to live under an oppressive theocracy.
@Mj:
When around 50 nations from the world come and occupy your land, then I will ask you how welcoming you remain to foreign agencies and foreign funded NGOs.
Are these the good taliban? Sickening display of barbarity.
Oops! Sick & tired of this war. I wish it to finish now.