The New York-based International Rescue Committee (IRC) confirmed yesterday that five Afghan staff members, all men in their 20s, were taken hostage on Sunday and killed by their captors the following day.
A sixth man killed was a local Afghan government representative who was travelling with the IRC team, while separately, six truck drivers who had been taken hostage were also found dead on Monday evening.
The spate of killings came as Karzai ended a two-day trip to Pakistan, calling on Islamabad to help set up peace talks to end 12 years of conflict in Afghanistan.
"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.
Nevertheless, Karzai invited Pakistani Prime Minister Nawaz Sharif to visit Kabul as the Afghan leader vies to overcome a series of public rows.
Infuriated by the opening in June of a Taliban office in Qatar, considered a precursor for talks with US officials, Karzai now wants Pakistan to help open dialogue with the insurgents.
The pressure to negotiate a peace deal with the Taliban is mounting as NATO-led combat troops wind down their activities and prepare to leave Afghanistan by the end of next year.
The Taliban, who ruled Kabul from 1996 to 2001, regularly target government employees in their nationwide guerrilla war against the Western-backed administration in Kabul.
Officials said the IRC workers were returning after holidays to Gulran district in the western province of Herat to continue work with the National Solidarity Programme (NSP), 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.