Two Finnish women working for International Assistance Mission (IAM) were shot dead in Herat, western Afghanistan on Thursday, the provincial governor said.
Governor Sayed Fazlullah Wahidi said the women were travelling in a taxi when they were shot at by men on a motorbike.
No group has yet claimed responsibility for the killing.
IAM has worked in Afghanistan since 1996, and is the longest-serving NGO in the country. Its staff, who all work as volunteers, serve on a number of medical, educational and sustainable development projects.
In 2010, a team of 10 IAM workers were killed by gunmen in Badakhshan, in north-eastern Afghanistan.
The UN has also reported that three of its staff working for UNRWA, its humanitarian organisation in Palestine, were killed by Israeli air strikes on Thursday. They were the first humanitarian fatalities in the recent violence in Gaza.
The number of aid workers attacked and killed in the field has increased in recent years. The Aid Worker Security Database reports that155 aid workers died in 2013, 85 more than the previous year, and the highest number in the last decade.