The five were ambushed and shot to death on Wednesday as they drove through southern Helmand province, according to senior provincial official Ghulam Muhiddin.
They were working on a US government-sponsored project providing alternative livelihoods to farmers growing opium, the raw material for heroin, Carol Yee, a representative for Washington-based Chemonics International, which is managing the project, said.
On Monday, four armed men dragged Italian Clementina Cantoni, 32, from her car in Kabul, where she works for CARE International on a project helping Afghan widows and their families.
Her purported kidnapper threatened on Wednesday in a telephone interview, broadcast on local television, to kill her unless the government met his demands, which included more Muslim boarding schools being built and authorities
helping farmers find alternatives to growing opium.
"If our demands are not accepted ... we will show our reaction and finish her," the man, who called himself Temur Shah, said on private television station Afghan Tolo.
He claimed Cantoni's health was "very critical", saying she had internal bleeding, was vomiting and had not eaten in three days.
Aid worker Clementina Cantoni
was seized on Monday in Kabul
Shah gave no proof that Cantoni was in his captivity. The Afghan government did not immediately comment on his demands.
CARE said in a statement it was not at liberty to comment on any aspect of the case out of concern for Cantoni's safety.
Authorities have said they suspect Cantoni was kidnapped by the same criminal gang that abducted three UN workers last year. The three were held for nearly a month, then released.
TV presenter killed
In a separate incident, armed men shot and killed an Afghan woman television presenter on Wednesday, two months after she was fired from her job on a pop music programme after complaints from religious conservatives.
The woman, Shaima Rezayee, 24, was killed in her home in Kabul, police said.
"We don't know who did it or whether it was related to her work or not," city police official Adbul Khaliq said. Police were investigating.
"If I win, I will work for the peace and development of Afghanistan"
Wakil Ahmed Muttawakil,
Taliban-era Foreign Minister
Private television and radio stations have mushroomed in Afghanistan since the ouster of the hardline Taliban in 2001.
Rezayee worked for a new channel called Tolo TV, which has won many young urban fans with its Western programmes and trendy presenters but also drawn criticism from clerics.
She presented a music programme called Hop before she was dismissed in March.
Meanwhile, a former foreign minister for the ousted Taliban government has said he has enrolled as a candidate to run in September elections for the new 249-seat legislature.
Wakil Ahmed Muttawakil, considered a relative moderate, surrendered to US forces in 2003. He was detained at a US base before being held under house arrest in Kabul,
Muttawakil (L) was the moderate
face of the Taliban government
and freed in April.
"I am an Afghan and I have the right to be an independent candidate," he told The Associated Press. "I am doing this for the sake of the people of Afghanistan. If I win, I will work for the peace and development of Afghanistan."
When asked if he still had any Taliban ties, Muttawakil said: "The Taliban are also Afghans. The public must decide who they want as their leaders, whether it's the
Taliban or someone else."
The head of the joint UN-Afghan election commission, Bismillah Bismil, said he could not confirm that Muttawakil had enrolled.