The attack is the second in two days against Western groups helping to rebuild the war-shattered country.
The nationality of the victims and the name of the relief group were not immediately known, said Ali Khail, the spokesman for the government in Zabul province.
The seven were driving on the main highway linking the nation's capital, Kabul, with the southern city of Kandahar when they were ambushed in Zabul province's Shahjoy district, he said.
Afghan police and soldiers have been deployed to the area, the official said.
The attack came a day after suspected Taliban fighters ambushed and shot to death five Afghans working on a US-funded project to help end opium farming in the south of the country.
Fighters have stepped up attacks on military and civilian targets after a winter lull, further impeding aid to the impoverished region.
Aid workers have been targeted as part of a drive to undermine recovery under the US-backed government that replaced the hardline militia.
Aid worker hostage
A man claiming to have abducted an Italian aid worker in Kabul threatened to kill her on Thursday unless his demands were met.
Clementina Cantoni was abducted
"The deadline we had given yesterday runs out today at 10am (0530 GMT) which is in one hour and we might kill her," alleged abductor Temur Shah told AFP, speaking from aid worker Clementina Cantoni's cell phone.
Local media have named Shah as the suspected kidnapper, but officials have yet to comment on the suspected abductor's identity.
Cantoni, 32, who works for Care International, was dragged from her car by armed men in the Qala-e-Mosa district of Kabul on Monday evening.
Shah called on the Afghan government to ban alcohol and a television music program, The Wednesday Night.
He also called for more support to be given to the country's madrassas, or Islamic schools.
Shah said the first deadline had been at 7pm Wednesday and the kidnappers had extended the deadline to Thursday morning and would not extend it again.
The alleged abductor added that Cantoni was feeling unwell.
Care International said Cantoni had been a humanitarian aid worker for 10 years and had lived in Afghanistan since March 2002.
Since September 2003, she has managed a project that provides food and income-generating activities for 11,000 widows and their children.